I'm not sure why a nat20 is a critical in a roll under? maybe making it "target number" exactly would work better for a "roll high but roll under" system? It just feels weird to my brain to have (for example) 12 be a success, 17 to be a failure, and 20 to be super good.

While this could change and feels wrong for a lot of people with roll under experience, there is something about the Nat 20 in the cultural zeitgeist that makes me wanna use it, and it has an interesting interaction with disadvantage meaning you are likely to fail but could get a critical success. It is in playtest but I super appreciate the feedback and I kinda go back and forth in my head a lot.