Using -forcehighpoly does not work well with some versions of Proton and Proton-GE and I would not recommend since the game already runs slow on occasion on its own.

Ensuring that esync is properly configured on your computer dramatically impacts this game. Without adjusting the limit the game will run in a slower and less stable state often resulting in a crash during longer games. Attempting to use mods without raising the limit is a non starter. Following the esync guide for Lutris is a great way to ensure that your system is functioning correctly and can be found here: https://github.com/lutris/docs/blob/master/HowToEsync.md

To install mods the most consistent and easiest way is to install and launch the base game as you normally would. After closing the game out navigate to the install folder and rename "Soulstorm.exe" to "Soulstorm.exe.bak" or any other name. Next after downloading the mod you wish to install move the exe into your "./Dawn of War Soulstorm" folder and create a copy that is renamed to "Soulstorm.exe" and launch the game as you would normally. This should bring up the installation for the mod and you should then follow the instructions for your specific mod. After installation is finished delete the mod installer called "Soulstorm.exe" and either repeat the process with other mods or create a copy of your "Soulstorm.exe.bak" and rename it "Soulstorm.exe" and launch the game as normal. Copies are suggested to reduce the need to wipe the folder and uninstall/reinstall the game since modding it is highly finicky. Alternatively copying a complete and working modded installation from a windows machine does work. -nomovies for less hearing loss when starting the game and not necessary to run the game.