(A lot of what i say about the Mesoamericans is almost entirely based around the immensely informative helpful posts of a 4chan /his/ board user known as Meso guy.
https://desuarchive.org/his/thread/5664811/#5666756 His posts were extremely useful over the days of research i've been getting into. I'd highly encourage anyone to check out his wonderfully made breakdowns of the Aztec civilization.
I'm not exactly sure how Tenochtitlan would be capable of ENFORCING such a monopoly, given they don't have a navy, the Spainish government in the New World (such as it is) has a loose grip at best on the behavior of the locals (Cortes himself is a perfect example of it: the man WAS slated for arrest by the Governor of Cuba while he went about his RL timeline, and it diden't slow down his actions at all. In fact, he managed to turn it around into reinforcements), and their base of power in the Valley of Mexico is hardly in a good position to keep a close eye and tight hand on the coastline particularly given their indirect style of ruling. I have no doubt some enterprising "commercial conquistador" is going to realize they can make literal boatload of gold by smuggling European goods along the coast, and given the trade deal with the Aztecs is by no means going to be as individually lucrative to ventures as OTL's plunder and seizing the production of the goods themselves approach that's going to be looking awful tempting as a "get rich quick" scheme. The only way the Aztecs would be able to prevent this is, more or less, bidding up the price in the offical trade ports so as to essentially pay a premium for the exclusivitiy, which is going to result in a greater flow of resources out of the Imperial treasury, which would mean an increase in required tributes from their subdued vassals... And I don't think its controversial to say that tax revolts are one of the most common causes of revolutions against the system in history.
And, assuming Spain has rivals in the region (Which they will; hiding knowledge of this is going to be difficult) those folks would be more than willing to sell to potentially rebellious groups so THEIR favored city can either be freer to buy more from their, protect/project their regional interests, or get on top of the system so they can kick out the Spainish as the main controller of the flow of goods. Add that to the political decenteralization/de hyper-ubranization created by the introduction of pandemic illness, and I imagine the system that's likely to develop is one of regional hegemons forming around a series of more equally-sized medium alliances of city-states with assorted patronizing factions (either other nations, or rival political forces in Spanish America). The Aztecs may still have a plurarity of power, but other cities would check them from re-establishing total hegemony.
Your comment and insight is exceedingly useful, i really want you to know that going forwards.
Alright, so Aztec controlling their vassals via local monopoly of goods is frankly an impossible scheme even assuming they would have wanted to do that. On further introspection it's also pretty obvious that the one's who would really be getting access to all the initial trade rewards with European powers would be the coastal cities with the interior cities and the Aztec alliance in particular getting their shares through normal idea spread & their vassal payments respectively. The Aztec domain while not the most stable of system wasn't exactly teeming with rebels for Europeans to bcack. The Aztec empire was militarily expansionist, to be sure but it really was a different system than what we know of today. Each city still maintains it's own rule, as long as you paid the tribute and didn't make a fuss you were left to do literally whatever: The ruling gang of cities didn't enforce many laws, rules, etc. And while the Aztec empire almost certainly would have fraactured, it really wasn't naturally on the brink of doing so when Spain arrived. There were some tensions, but had the Spanish not shown up it's likely they would have been resolved as normal: Namely Tlaxcala would have fallen to Aztec sieges and been absorbed. I also don't really think the plague crisis can reach how bad it was in OTL...The OTL epidemic killed 1/3 of the population over 10 years before it started to slow down. OTher pandemics followed it in the decades after. Without Spanish occupation a lot of outbreaks do not occur or aren't going to be 'as bad'. It should also noted that native social structures did actually survive the arrival of OTL's smallpox, even if there was considerable loss and disorder in the process. Spain had to take the next step and spend most of the sixteenth century breaking apart Native social orders by force.
It's hard to really say for sure exactly how bad diseases will turn out to be in the long run this time around and yeah, it'll still be *really* fucking bad as an unavoidable sweeping tide death but things really should be a good deal better off without Spain doing their conquest overtop of it.
On the matter of Spain & European parties partaking in raiding opportunism all i will say is that i fee you are letting your perceptions get clouded by OTL. Spain's policies and behaviors during and after their conquest of mesoamerica were radically different before Cortes started sending back his new world plunder. Cortes was very nearly executed for treason after the fact in OTL for provoking a war which he won and now somehow ittl where Spain should be raking in easy money and be scrambling to expand their list of contacts with other native groups along the Mesoamerican coast there are going to Spanish opportunists trying their hand at potentially dealbreaking raids? Do keep in mind that the really The comically insane successes of Cortes set the mood of Spain's strategies OTL and i don't see why it wouldn't also set the spanish mood here as well.
Imo it's not like i think having rampant European hostility would really be a bad thing for the Aztec/Meso groups. I don't think its controversial to say that powerful foreigners going on a string of raids against your frienemy neighbors are one of the most common causes of uniting otherwise divided groups into allies in history.
Let's start from the beginning: "things go over wonderfully for everyone" means that the Aztecs are happily continue with their nice habits of the massive human sacrifices taking these sacrifices from subdued tribes (who were seemingly hating their guts) so I don't think that you meant "everyone" literally.
At least in OTL immediately after the initial successes against the Aztec vassals and well before he reached the Aztec territory Cortes had been ordering the defeated tribes to swear loyalty to the King of Spain so this has to change as well in your scenario.
As for the rest, Cortes and his expedition were not traders: by his background he was a military person and the same goes for most of his band. Anyway, trading as an occupation was unacceptable for a Spanish noble so I'd assume that there was not too much of an expertise in how to establish the trade relationships and I'm not sure if the Aztecs had been too much into the trade either. The Spanish troops in Europe had been the famous looters while the Spanish merchants were rather rare beasts and Spain did not have too many items to trade to start with. What would they offer to the Aztecs? Surely, selling them weapons would be a little bit too much on the idiotic side so what's the list of the Spanish goods in which Aztecs could be interested?
Availability of the gold and gemstones in combination with a clearly inferior native warfare indicated a good looting area, not a potential trade partner and departing with the gifts (a big part of which would go to the king) would just mean that the members of expedition are not going to end up getting rich (I suspect that many of them did not, anyway) while somebody else is going to benefit in the future (again, this eventually did happen in OTL but it was not the goal of the expedition). After all, the expedition was a private enterprise and its members were not looking for a long-term "everybody's" gain. Conquering the territories in the name of a king was just a way to legitimize their actions.
Yeah, the premise as i wrote it there is built on flowery language more than being super literal or serious but i do want to make sure i have everything hashed out on the historical accuracy front before i try making a timeline out of this. The short and simple is just that the spanish under cortes party and have fun during their time in montezuma's palace and not instead use it as a stepping stone to ride out an inconceivable conquest on the sails of good luck.
Now up till now i've been very much been theorizing under the impression that Spain's goals on the world stage at the time were to ape the successes of the Portuguese. Cortes had been acting way out of line from what he was supposed to be doing from the start. Ignoring the governor's command to stop, scuttling his ships after landing and pillaging the first tribes he found . The spannish sent men to arrest him for a reason after all. If i really am that wrong about his & (much more importantly) the Spanish mindset as a whole at the beginning of this then i suppose i'd just sort of need to accept that the premise is asb bunk on the simple fact that Spain's reason for sending expeditions was to scout. Simular events can follow if any of the more incredibly lucky things Cortes did ended up blowing up in his face
"What's the list of the Spanish goods in which Aztecs could be interested?" Um...anything and everything that can be found in europe really, metal tools and instruments, exotic fabrics, alien foods, old world livestock. Trade standards between the natives and colonialists really were silly. The British and french managed to run a massively lucrative trade system on the basis of trading things for animal pelts of all things. I doubt the Aztec would have much issue in that field when they have such exotic luxuries as Cocoa & pepper spices among the stockpiles of gold.
"I'm not sure if the Aztecs had been too much into the trade either." The mesoamericans were avidly into trade. Canoes and boats were used extensively to transport goods around the lake from cities and towns and the tributes usually involved a degree of bulk shipment. The markets of Tenochitlan were probably larger than the ones of Constantinople and the Maya just next door were even more invested in trading. Trade exists. It's not an alien concept. I agree in retrospect that weaponry sales would be inconceivable in bulk during the initial years but i will say that those things have a tendency to pop up with proximity.