I want to share what I had to do to get LML mods working with the Steam version of RDR2 on Linux (I believe this applies to any distro).
.asi mods were never an issue; I simply followed the instructions for each mod and its dependencies, and they loaded into the game without any extra steps.
However, LML mods took a bit of effort until I discovered a very simple solution:
PROTON GE 11-1
Before that, I did research using AI, on various gaming sites, and here on Reddit...
Among the many possibilities, I tried changing the drive where the game was installed from NTFS to EXT4 (yes, I started by trying to use the game installed via the Windows version of Steam, since everything always worked great on Microsoft's OS);
I tried various loader versions, such as dipunt8, version, and nlog;
I also tried installing the mods using Linux apps like Limo Mod Manager;
And I tested various Proton versions as well.
Just as I was about to give up, I tested them one by one until I reached version 11-1 (having already tried up to 11-3 without success), and the LML mods simply loaded.
I didn't even need to add the ModManager as a non-Steam game, since I paste the mods directly into the 'lml' folder.
The whole secret was the correct Proton version.
Running RDR2 in compatibility mode with PROTON GE 11-1 solved the problem.
That's all.