Solus will install in UEFI mode if you boot via UEFI, and legacy (BIOS) mode if you boot via legacy. Not really sure what's going on there.
If you're not too worried about your data, encrypting your drive is overkill. you'd have to enter an extra password every time you boot the computer, and I'd imagine there'd be a slight speed penalty when reading/writing the disk (although if there is, it's not noticeable).
as I understand it, LVM is good if you want to resize your partition later or extend it across multiple drives, but if not, then maybe it's not necessary?
I don't think you need to worry about a Steam game "breaking your OS". at worst it might crash or freeze, not that I've experienced much of that, especially with games designed to run on linux 🙂
