Bazzite, it's an immutable Fedora-based distro, so in the unlikely event that it breaks, you can just revert back to whatever you had before.

Nobara is similar, Fedora-based but not immutable, which means you can tinker with it, but possibly also break it. Made by Glorious Eggroll, the guy behind the GE versions of proton and wine.

Mint is a more general-purpose distro, based on Ubuntu (which itself is based on Debian), but it's very user-friendly and does just fine with games.

Manjaro is fine, it's the one I put on my mom's computer because she needed a Windows program that I found in the AUR. It was pretty decent for the four games that she plays lol (The Sims 4, AoE2, Neverwinter Nights, and Prince of Qin). It's Arch-based, but not bleeding-edge like Arch, so it's ostensibly more stable.

As far as the Index goes, idk about that, as I don't own one. However, I just DDG'd "valve index on linux", and quite a few guides came up, so it shouldn't be too hard to get it going. Plus Valve is a pretty Linux-friendly company,