I play on a steam deck, is that problematic to submit runs? I can record gameplay + inputs etc as far as I know

Yeah its fine as long as you are following all of the same rules. Just keep in mind that the game currently isn't entirely supported on linux so some levels (namely 5-S) may* crash.

5-S crashes on Linux? I'm fairly certain I beat the entire game on Linux, so that probably more depends on which distro you use or the hardware you use. I don't think I've ever seen the game just crash on me suddenly.

Edit: I have seen 5-S lag a lot after some time, so maybe that has something to do with it? I've mostly only played it around the time it released though.

5-S works just fine for me