To answer the questions directly, no, Darkrai isnt too powerful and the Gen 9 Pokémon from 18-21 seem okay enough. The issue lies with, well most of the other Pokémon. To name a handful of examples, Zygarde-Complete Marshadow, Mega Salamence, Mega Lucario and Mega Kangaskhan are all objectively broken in a semi-Ubers format, and should be both far more expensive than, and in the ban discussion ahead of, Darkrai (there's definitely some other Pokémon that fit that criteria too but I just mentioned the first few I saw). Beyond that, your placement of a lot of other Pokémon, for example, Magearna and Pheromosa (both of whom are strides better than the rest of their tier and even many of the Pokémon in the tiers above) are incredibly low.
I'd suggest looking at the tier lists from some other NatDex leagues to draw some inspiration for your own tiering, I'll link a few that come to mind. While they're a few weeks-a month or so out of date at this point, they should be able to give you a general framework to based yours on
PWS - check NatDex Draft Board here
Colo - check Draft Board here
WPF - check Ice/Rock Draft Board here
WPF Rock will most likely be of the most use to you, it's an Ubers league, you can very easily take their tier list and cut it off at around 23-25 and then move Pokémon around to fit your format.