It's been a while but I think I also struggled to hit a solid 60 without substantial graphical downgrades. The biggest thing to check, if they haven't fixed it, is if you're using proton? The Linux version of the game, at least as of last year, is total crap. Running the Windows version through proton is a substantial improvement. Hopefully enough to get you to 60fps.
As replied in another comment, I switched to Proton 8.0 and can now reach fixed 60. Playing at high preset and manually maxed out environment quality, weather quality and population.