So this is what worked for me with fresh installs on both Ubuntu 21.04 and Manjaro with gnome (aug 13 2021) and nvidia 470 driver:

• Grab a coup of coffee. Maybe some chill music also.
• Install steam.
• Install Supreme commander: forged alliance using steam.
• Enable steam play (steam -> settings -> steam play -> "enable steam play for all other titles" and i set it to Proton 6.3-5
• Launch options: In steam library, right click Supreme commander: Forged alliance -> Properties...
  Then under "General" -> "Launch options", put PROTON_NO_ESYNC=1, PROTON_DUMP_DEBUG_COMMANDS=1 %command% /windowed 1024 758
  Under "Compatibility" check "Force the use of a specific Steam Play compatibility tool" and choose whatever Proton version you think is best. I went with 6.3 but some ppl say 5.0 is better.

You can try to run the game now, it crashes on launch for me, saying
"CD3DDeviceResources::DevResInitResources: Unable to load effect file /effects/cartographic.fx"

If game runs fine, skip the next section and just edit game.prefs (last step)
If you get that error, close the game and let's fix it.

• Install winetricks (it will probably also install some dependencies and wine itself)
  ubuntu: sudo apt install winetricks
  manjaro: sudo pacman -S winetricks

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

• 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 should start. 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%.

Rune game again and try skirmish.

If you use analogue headphones and have no sound, try change sound output in ubuntu/manjaro gnome from "headphones" to "line out". I've never had any issues with this before, must be some Wine edge case.

• All that is left now is to install faforever client but i'll leave that to the wiki https://wiki.faforever.com/index.php?title=Setting_Up_FAF_Linux#Java_Downlord.27s_Client

So this is what worked for me with fresh installs on both Ubuntu 21.04 and Manjaro with gnome (aug 13 2021) and nvidia 470 driver:

• Grab a coup of coffee. Maybe some chill music also.
• Install steam.
• Install Supreme commander: forged alliance using steam.
• Enable steam play (steam -> settings -> steam play -> "enable steam play for all other titles" and i set it to Proton 6.3-5
• Launch options: In steam library, right click Supreme commander: Forged alliance -> Properties...
  Then under "General" -> "Launch options", put PROTON_NO_ESYNC=1, PROTON_DUMP_DEBUG_COMMANDS=1 %command% /windowed 1024 758
  Under "Compatibility" check "Force the use of a specific Steam Play compatibility tool" and choose whatever Proton version you think is best. I went with 6.3 but some ppl say 5.0 is better.

You can try to run the game now, it crashes on launch for me, saying
"CD3DDeviceResources::DevResInitResources: Unable to load effect file /effects/cartographic.fx"

If game runs fine, skip the next section and just edit game.prefs (last step)
If you get that error, close the game and let's fix it.

• Install winetricks (it will probably also install some dependencies and wine itself)
  ubuntu: sudo apt install winetricks
  manjaro: sudo pacman -S winetricks

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

• 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 should start. 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%.

Rune game again and try skirmish.

If you use analogue headphones and have no sound, try change sound output in ubuntu/manjaro gnome from "headphones" to "line out". I've never had any issues with this before, must be some Wine edge case.

• All that is left now is to install faforever client but i'll leave that to the wiki https://wiki.faforever.com/index.php?title=Setting_Up_FAF_Linux#Java_Downlord.27s_Client

@pa4sam this post, in the reddit thread you linked, helped me:

the_bois:

Hey! I just had this problem and the solution was not obvious. I don't understand the whole complexity of it, but for me it was missing some libraries (notably d3dx9) that are not bundled into proton by default (I imagine because it is so old!).
To fix this I installed proton tricks following this guide (https://www.simpler.website/html/2021/1/protontricks.html) and then installed the d3dx9 library with the command
"protontricks 9420 dlls d3dx9"
I also needed to run
"protontricks 9420 dlls xact"

This was on a fresh install of Ubuntu 21.04 with nvidia 470 drivers. But i tried absolutely everything (nvidia drivers, different proton, reinstalled steam, even reinstalled Ubuntu, tried every 386 driver i could find etc) in every thread before this and nothing worked. Game crashed on launch. But i could start the game fine after these tricks.

Edit: if you change proton version, you need to redo this, (and maybe recopy the run file)

Edit 2: actually i seem to have no sound at all ingame...

Edit 3: lets give an update. I fixed the sound simply by changing "headphones out" to "line out" in Ubuntu sound settings. Dunno why it worked, i have never had issues before. Maybe something with xact and proton and gnome and pulseaudio... I use old analogue headphones, without USB connectors.

Anyway i found a spare drive and installed manjaro, fresh install steam and forged alliance, and get the same errors. Only installing d3dx9 and xact fixed them. Sample size = 2 but i think those libraries are missing from proton. (I also had the exact same audio problem in manjaro).