If you go with kubuntu you'll be using snaps by default (which can be removed entirely with some tweaking) and they aren't actually good (as with the recent steam issues)
I'm not trying to steal hollyberry's job here but here is my understanding of snaps (and why they aren't good).
Snaps were created by Canonical (The company behind Ubuntu) to fix the issue of inconsistent dependencies. The problem with the format is that the market is proprietary and they just aren't very good. Also they perform somewhat worse than Appimages and Flatpak.
Personally I reccomend you look into Flatpak, as it's a better sandboxing format than snap is.
Also the reason you ended up with the SNAP version of steam is because Ubuntu prioritizes the snap version over the native version when using
The moment steam ends support for win10, is the moment I fully switch to Linux.