It worked, Thank you!

Before I stated I installed the open source AMDGPU driver for my RX 570 with pretty much everything from the "Install" section: https://wiki.archlinux.org/title/AMDGPU
I am on Manjaro, the only problem I encountered was that the game crashes on launch, saying “CD3DDeviceResources::DevResInitResources: Unable to load effect file /effects/cartographic.fx”

I followed this guys guide: https://www.protondb.com/users/1007765403
But skipped the resolution changes. Maybe that was why the first start of the steam version took quite long, the cut scenes were laggy but skippable.

Install winetricks:
ubuntu: sudo apt install winetricks
manjaro: sudo pacman -S winetricks

Install protontricks using this guide from simpler: https://www.simpler.website/html/2021/1/protontricks.html

Copy from simpler:
Arch based Linux distros:
Open the terminal, then copy and paste these commands:
sudo pacman -S python-pip python-pipx python-setuptools python-virtualenv
python3 -m pip install --user pipx
python3 -m pipx ensurepath

Close and reopen the terminal again.
Install protontricks using this command:

pipx install protontricks

in a terminal, write: protontricks 9420 dlls d3dx9 in a terminal, write: protontricks 9420 dlls xact

(if you, at a later stage, change proton version in steam, you have to reinstall the above two libraries since they don’t seem to come with proton.)

Now start the game. It took quite some time for me. Create a game profile ingame and exit.
edit your Game.prefs on ubuntu it’s located in:

~/.steam/debian-installation/steamapps/compatdata/9420/pfx/drive_c/users/steamuser/Local Settings/Application Data/Gas Powered Games/Supreme Commander Forged Alliance/ change primary_adapter = ‘1024,758,60’, to whatever your native screen resolution is, mine is: primary_adapter = ‘2560,1440,60’, and save.

Edit launch options again (see step above) and remove “/windowed 1024 758” leaving just PROTON_NO_ESYNC=1, PROTON_DUMP_DEBUG_COMMANDS=1 %command%. All that is left now is to install faforever client (for multiplayer), check the wiki: https://wiki.faforever.com/index.php?title=Setting_Up_FAF_Linux#Java_Downlord.27s_Client