So I did look more into it, and apparently the open firmware is technically compatible with PCIe cards using this chip, but doesn’t provide any advantages over just wiping the firmware and letting the chip default to its built-in fallback firmware, and so the maintainer doesn’t see any value in explicitly supporting it.
Now the question is whether you consider the proprietary fallback firmware to be acceptable to run - this might sound weird, but for example FSF has explicitly made exceptions for devices with built-in firmware to be able to qualify for the Respects Your Freedom certification, so if your view aligns with theirs, you might consider this to be completely OK. If not, the free firmware appears to have a similar feature set, you’ll just have to jump through more hoops.
Also do note that both the fallback firmware and the free firmware are missing many features of the proprietary firmware, so make sure to check it’s not missing anything you need (wake on LAN, Jumbo frames and PXE boot seem like the most notable missing features to me).
xrandr is Xorg only, it doesn’t work with Wayland. You should be able to make SDDM use your Plasma display configuration - https://wiki.archlinux.org/title/SDDM#Match_Plasma_display_configuration
No clue if that’s going to fix your issues, but at least it’s supposed to work with Wayland.