I pretty much turned everything down and the render scale didn't help that much weirdly, I didn't see whether the CPU or GPU was maxed out which is mb, but it definitely wasn't a memory issue.
Also the steam deck does have some pretty good hardware.
Edit: I tried to get back in the game to measure what it was stressing but all 3 times (2 from main menu, 1 after the tutorial) crashed, one of them even crashed my computer, it might be a GPU issue in that case.
I don't really know why I expected to run a shader heavy game 😅 Sorry!