I know AL high level modules were mentioned, but does anyone else have suggestions for a good high level module on the DMs Guild? I'll certainly shell out the money...
That is my largest complaint about the DM's Guild...I don't know if there's a way to search by level for the modules on there.
No suggestions other than to reiterate the fact that writing high-level adventures (T4, specifically) is kind of hard to do because of the way WotC designed the CR system, and the way magic (or the lack of it) can not only change the challenge by also bypass it entirely.
Like, an Ancient Red Dragon could be a TPK for a group of level 20's that aren't decently optimized (or the DM uses appropriate tactics for such a creature), but could also be killed by a well-organized group of level 15's, depending on the class makeup. That kind of variation is tough to plan for when your writing an adventure with no clue what the size and makeup will be of the party.
Or how do you write an adventure that has, say, 8 encounters, knowing that any number of them could be totally bypassed if the group can fly or teleport around -- possibly without ever knowing those encounters existed in the first place. To do that, you have to either restrict the usage of such spells and resources somehow, or structure it in a way that the group really just zips around from one Important Boss Fight to the next. If they can teleport, cool; if not, just paraphrase the journey. Either way your design space is a bit limited.
For these (and other) reasons, the high level adventures that you do stumble upon, are often designed to be vague pseudo-political romps where you RP with the elites for a few minutes before zipping over to the next boss fight, or they are essentially just high-CR encounter grinds with some half-assed plot to justify why you have to go kill 4 ancient dragons, a lich, and few pit fiends, and tarasque (and of course balancing it all without a clue as to what the characters are capable of).