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@wouldbebard
Just a couple of things on the "Unrelated to the above" part:
[Disclaimer for wouldbebard, just my opinion, doens't mean I'm right, and I hope I don't come off as passive-agressive to you or argumentative. I love you a lot <3]
In order to provide challenge to higher levels you do not require to make hordes of creatures or make aboleths/dracoliches a common thing, at all.
I believe it's the mindset here, that only player PC's are leveling up. In the same fashion there's humans/elves/etc that reach level 20, there can be NPC human/elves that reach level 20, and so they'd be in equal conditions when fighting the PC's. In fact they already exist in the DM spawner cause I created them.
As long as things are laid out in the Narfell map progressively (so that a level 10 doesn't face a level 20 spawn by accident) it shouldn't be a problem.
As a DM, also, I've never had a problem challenging high level PC's in my events without needing balors/dragons type of spawns. One just has to learn how to, I suppose.
Your idea also seems to be based on the premise that the server gets a full reset, or else you would have to delevel people's PC's. And that generally doesn't sit well with players. (Yes, I do have PC's that would need deleveled and I'm not particularly happy with the idea either, eventhough that wouldn't make me stop playing, probably).
More than downing the top levels I would slow down the level up curve.
It took me 4 years of playing an awful amount of hours to get to level 17 with María. I think that's reasonably slow that you wouldn't need to worry much about cramping the top levels. Specially if you take out the TR.
Nowadays you can get a character to level 16 within a month, with 0 DM events XP, and without the help of any other player's characters. THAT is what needs to be adressed.