Lots to respond to!
Think most important things are already said save for logistics and cultural differences.
The larger an empire becomes in a short time the more the people will resist. Especialy if its armies are spread all over the place. And local leaders see opertunity revolts happen and rebellions spread.
The cultural difference between the greeks and the persians was so big that the greeks would be easily aiding any rebellion. That means you need troops stationed all over greece to keep every city in check.
Taking into acount that the Athenians kicked out the spartans earlier they wouldnt take kindly Persian overlords either. So expect rebellion everywhere in greece. Just like in Egypt. Also the logistics at the time dont favor a big landmass (like said earlier). So Persia at the time was realy at its peak, actualy it went to fast. Same with the greek/macedonian empire Alexander created. You cant keep a large landmass together like that unless you change hearts and minds of people in your favor.
I'm pretty sure I mentioned logistics in my post. Even so, taking care of the Greeks is easier than others. Why? Because the Greeks have internal feuds that can be played upon. When Persia conquers Greece they will most likely institute a tributary/satrapy joint system, depending on what the city state is. Sparta and Athens will both be razed to the ground the Helots will be given Spartan land. this will appease the Helots, which control most of the area. The Laconians will most likely be free to rule themselves and be given former Spartan land.
Cultural differences don't mean much. Persia controlled an empire of so many different cultures and nationalities that at this point I don't think they had a large problem. They would appoint royal princes to the Greeks as Satraps, make them fork over Hoplites for levies and as mercenaries in the Spada. It's quite easy to get their hearts and minds in your favor by bringing a much more prosperous economy and introducing
Qanats to Greece.
Even with their hearts and minds in the sense of them not rejecting Foreign Barbarians, you still have the local big shots deciding that the emperor is far away and they control enough of the army to establish their own polity.
I don't know if Persia had maxed out its available growth (as in, it couldn't take any more without crippling costs or defeats), but it sounds like it was already in the range of "the most it can feasibly hold on to" - that is, the territory within which it can project power against regularly enough and successfully enough that if and when troubles come from either locals or overmighty satraps that central authority can be reimposed.
Egypt is doable, if problematic - but its on the frontier between the realm of more-or-less secure to more-or-less independent. Any further west would be getting increasingly "Okay, you can conquer it, but can you hold it longer than it takes the army to about face?" even without any particular unruliness - the sphere where central authority is acknowledged has reached the limits the technology and geography permit.
My impressions, some based on other statements here.
Again, Persia wouldn't go further than Egypt. The only reason Egypt looks problematic in OTL is because of the fact that the Persians had to contend with Greeks in Nacrautis stirring up trouble and Athenian funding for Egyptian revolts. Without that Egypt is easy to control as the Persians simply appealed to the Priests and showered them with privileges and the people were happy.
Well the only way you can successfully build a bigger empire is through doing it slowly, as in converting the subjects you conquer into *persians* themselves and gradually move up. So that the conquered no longer feel like being subjugated. But this takes time and what the persians mostly did is conquer and move on to the next. This is what gave rise to revolts in the first place. You cant enslave a people and then leave a token force behind to deal with uprisings and move on.
Then comes the vast distance that at the time was impossible to travel. By the time the persians had a army in the field the rebels would have formed government and their own army, making reconquering thus harder then the initial conquest was. Cause this time they aint fighting for a king or regent. (lets say the egyptians or the Ionians, but their own freedom.) The first test on this was the american civil war, trains were the revolution in warfare and you could deploy troops way faster in that time then the persians could do. Btw most the persians were infantry.
To my understanding all empires before inproved transport failed, Greek-Macedonians,Persians,French empire. You conquer to fast to be able to quell resistance behind your own army. So the Persians were at their max with what they had prior to the Greek invasions. Actually the Ionian revolt showed its shortcomings.
The Romans however held onto their empire due to its Mediterranean location. as you an see the distance into the north and east is rather limited (south is obviously desert). Britain they eventually gave up on. And the expeditions into both Germania and Persia failed, again logistical issues regardless of the battle tactics of themselves and foes. A war is usually won by whom wont give up and can pour in more troops.
Well im diverting a bit from the subject but just to give an example.
Still the size of the persian empire and its long lasting shows its remarkable. But any attempt on further expansion would be doomed.
Conversion? Zoroastrianism was a petty cult during this time, not in any way majority, not even large minority. One has to remember the Persian empire is an occupation that allowed the occupied cultures to continue living as they did with taxes and levies, and with the bonus of being more prosperous than they were before. Areas like Syria grew to tremendous levels of wealth and the Mausoleum of Halicarnassus, in a formerly poor area of Anatolia, attests to what Persian conquest could bring.
In Persia the point of a satrapy was to give a province both autonomy and reassurance that their culture wasn't under attack or that they had something to fear. When Cyrus allowed the Jews back to Israel it was because he wanted to make a political statement.
I'd also like to note that the Persians had several armies. Contrary to an accident in my last post, the Persians had three armies.
- the Professional army, or Spada. Composed only of Iranians and mercenaries it was the core of the Persian army.
- The Provincial levies. These were usually called upon in times when Persia wanted a Grand army to attack something such as what Xerxes did with the Greeks. They were brought from all over the empire.
- The Satrap armies. This is the small army of the Satrap in their territory, and was the army which Alexander faced in Granicus. This is a double edged sword. On one hand it stops rebellions faster, on the other hand it gives rebellious straps a lot more power.
And rebellious satraps were watched carefully by imperial inspectors using the famous royal road, making rebellion much harder.
I do not think that the Persian empire would be really interested in conquering additional portions of Europe, with the only possible exceptions of Sicily and Crimea, both being wheat producing countries.
The task of keeping together a really big empire is daunting, but not impossible. There is no large enough competitor (unless there is a big empire forming in India - even in such a case the logistics of attacking from the Indus westward are quite poor) and even if provinces can be temporarily lost due to insurrections or civil war they can always be reoccupied and pacified.
The only real problem is with the nomads coming out from the steppes, but it is a problem which will be real for any kind of state controlling the southern Balkans and/or Anatolia and/or the Iranian plateau. Darius had his own troubles with the nomads, and went on a famous sweep of the steppes which did not really produce any lasting success: the nomads will be a big real headache until the time of gunpowder weapons.
Funnily enough, I might forecast an east-west division of the empire similar to what happened in the Roman one: a WPE with its capital in Byzantium (surprise, surprise) and an EPE with its capital as usual in Ctesyphon. The border would almost automatically be set between Syria and Mesopotamia.
An interesting byproduct of a Persian conquest of Greece might be a significant Greek diaspora in the western Mediterranean, which might produce a much larger Magna Grecia and possibly a Greek Provence. No guarantees, however: it might go the other way too. Greek culture is very vital in this age and might permeate the Persian empire even without the boost of a Greco-Macedonian military victory.
It will most certainly not conquer the Crimea. They would need to conquer the land route to it and as recorded by Herodotus they were checked on the Danube. As with Sicily, again, too far as the Persian power would have already begun to fade in Greece. They don't grain producing regions; they have the fertile Persian provinces of Persis and Media in addition to the Fertile crescent. Besides, neither had really increased to the recognition they would receive later for their grain.
The Persian empire was already losing territories to Magahda in India when Xerxes was around. If we get an able emperor, he will have to battle the Magahdans for it back.
Nomads from the steppes consist at this point consist mainly of Scythians, which Darius drew(The Dahae/Parni at least) into a tribute system. If they do attack they will be driven off and they declined in the later Persian period anyhow. While Darius' sweep didn't do much good it did in fact negate the Scythians from attacking Xerxes.
There will be no east-west division. The problem is Persian culture itself is confined to the Iranian plateau and as we saw with the collapse of the Persian empire they never went beyond the Mesopotamian basin ever again. This is because they let the peoples in the area lay in peace. Unlike the Romans its impossible for such a divide to exist, and indeed, it contradicts traditional Persian claim to divine authority. There can't be two
Shahanshahs at the same time.
Two things may happen to the Greeks.
1. Many Greek soldiers will be campaigning a lot, spreading Greeks around the empire.
2. There will be more deportations to outer regions of the Persian empire. This seems quite likely and the destination, again, will probably be Bactria.
Also, Bystantion was a fishing village. It would be Sardis if anything. Ctesiphon was not founded until the Parthians, so I think the eastern empire would be in either Persepolis or Pasargadae.