I never made it to the D&D endgame. Okay, unless you count my adolescent campaign where all of us young boys ended up with god-like characters choosing wives from the Dieties and Demigods (I don't remember who I chose, but Loviatar had my eye for a while). But I don't count that, it was, in fact, an early schooling in how un-fun Monty Haul is. The main reason for this is my history with this hobby has been one of perpetually trying to play-- never enough people around to form a group.
Because of this, the highest level I ever achieved was about 8th as a player in our campaign that ended in highschool. I've never got to fight a beholder, or a lich, never got to research my own personal spells (I so loved the named spells in 1e).
When I did manage to huddle a few players around me I dealt a lot with trying to keep low level characters challenged and engaged in my campaign-- low level monsters, puzzles, weak magical treasures. In some ways this is very difficult, but overall I think it must be simpler than working with higher levels; to challenge a 10th level character you have to keep in mind the whole range of magic and abilities they are likely to have. Low levellers are barely able to bring spells to bear on a problem, much less have magic items.
So, one way I realized I could scare my characters without killing them, was aging (I kept strict track of age). That leads to my next monster, which I was very proud of thinking up, but alas never got to see in play. In fact, just recently going through my old maps, I saw three of the little guys penciled in where I had intended them to be encountered.