I never expected this plan to work, I just want to know what would've happened if it was launched.
I do imagine it would go down in history as one of the absolute worst military decisions ever.
Well, in that case, my take on it would be that the US would learn about it a few days after the fleet's departure. It's simply too large to be ignored or avoided. So the US would have about 3 weeks to prepare.
The Royal Navy would as well notice this, and would also prepare their own defenses in the new world. After all, there's no definitive target.
Both the US and the UK would summon the German Ambassadors, and both would be unable to provide a satisfactory answer. Either the Ambassadors deliver a declaration of war, spoiling the surprise, or they insist that this is merely a German Fleet exercise and demonstration and no need to worry, or they immediately request asylum as the Emperor is a madman who won't listen to reason.
Either way, the US and Great Britain both know something is up, so independently both will begin buffering their forces. Coastal forts across the Eastern Seaboard receive a flurry of effort to bring them back to modern standards. Thankfully, this already occurred a few years prior with fears of Spanish invasion, so the forts are in better shape than they might otherwise be. That's also determined if the US has a good idea of where the attack is going to be located. If it is known, then they can intensively fortify and bring artillery in to prepare for a landing in the Northeast. If not, they'll defend that region more than others, but the entire seaboard will be locked down, which means that some men and material would be diverted.
Secondly, the US and Royal Navies will be recalled to local positions in order to oppose the landings. They're not likely going to coordinate, but at the very least, both will have significant presence in the area. The US will try to combine into a single fleet to oppose the landings, again, depending upon the amount of time and whether they are aware of the location of the landing sites.
Either way, the US fleet will make a serious dent in the German vessels once it becomes clear a state of war exists and, more importantly, will likely damage some of the troop ships. Combine that with any bombardment done by ground-based batteries and torpedo attacks (especially nighttime/dusk/dawn raids), then it wouldn't be unlikely that a sizable amount of those troops never even set foot on the beaches, much less make it up unopposed.
Then there is the problem of where German vessels are to retire to when they are damaged and low on supplies. There isn't a single major port in North America that would take them in. At best, a heavily damaged ship in danger of sinking would either have to scuttle or risk making a run to a friendly neutral or, even, simply surrender and head to a US port, where the vessel would be taken as a prize and the crew impounded.
On top of all of that, the troops that make it ashore, assuming they make it past the beaches, will have taken heavy casualties and expended a ton of ammo. The only reason they're not overrun by a larger, less experienced and trained force is because they are under the careful eyes of their navy, which is dwindling by the day.
That's not to mention that the German ships can't even make it back! The Brandenburgs only had a range of 5200 miles. From Kiel to Boston, as the crow flies, it's about 3600 miles. Combine that with not being able to sail through England, along with maneuvers to avoid patrols, and it likely rises to 4000 miles. (Compare the older predreadnoughts, which only had a range of 1600 nmi or so.) There are going to be a ton of ships packed with coal on top, and even then, the HSF might not be able to make it to the Eastern Seaboard with a large portion of its fleet. That's an immediate strike against its power. At best, I imagine they were thinking the US would surrender, they'd get coal from the East Coast, and sail away? That isn't going to work.
So the troops can't even retreat to the sea, as none of the ships they came on have enough fuel to make it home. So, once it becomes clear that they can't break out and have taken excessive casualties, they will have to surrender. Some of the HSF might make it away and flee to neutral ports, or try, but I don't see how they manage to leave at all. And, in the process, New York and Massachusetts are heavily damaged, but not beyond repair, and the US have many, many PoWs with which they can bargain with the German Empire.
In the end, the German Empire can't repeat this, and the US is not going to launch a counterattack, so the US will try to extract as many concessions as it can for the return of whatever PoWs are left. Any German warships captured will likely be retained as compensation for American vessels sunk. All of Samoa likely goes to the US. Maybe the Marshall Islands will as well. But, more importantly, the US has been inexorably driven into the British camp, and the Germans lose a major component of their power projection abilities.
This is, of course, assuming that the British don't intercept the fleet and prevent it from landing in the first place.