

Ran perfectly with 6.3-6
Can't get past the Steam check. User Interknet's modified .dll file is no longer available at the link he posted.
If you used the previous method, navigate to
/mnt/LinuxHDD2/SteamLibrary/steamapps/compatdata/1154850/pfx/drive_c/users/steamuser/AppData/LocalLow/Valorware/9th Dawn III/SaveData
And move save files from the SteamID_SteamNullID folder to your own ID's folder (make a new char in-game to make it appear). Achievements seem to work fine now. No idea if cloud saving does as again, I don't use it.

You need this fix: https://mega.nz/folder/5sw3ja6A#La7oC7-96ZcONDc37sAYhQ
The fix may cause cloud saving issues. I don't use cloud saving, nor am I sure if proton supports cloud saving either, but bare this in mind.
Everything else works great with the fix and I'll try to keep it up to date when the game gets updated.

Must use the file.dll from Interknet user (here on ProtonDB) in order to play the game. No Cloud Save, but local save are fine.
No Cloud Save with the modified .dll from Interknet, but local saves are fine
Need to modify the game file: https://mega.nz/folder/5sw3ja6A#La7oC7-96ZcONDc37sAYhQ
You're welcome :)

Needs modification of the game files
You need to download the fix here:
https://mega.nz/folder/5sw3ja6A#La7oC7-96ZcONDc37sAYhQ
You're welcome :D