Didn't work with vers of Proton > 5.13.6. Game loads, but only displays red, no Green/Blue is shown. Works mostly OOTB on v. 5.13.6. Add'l tweaks recommended to fix resolution issues. My entire command string: PROTON_FORCE_LARGE_ADDRESS_AWARE=1 %command% -CustomResolution:enabled -r1600x900x32 -f -Cursors:fullcolor -Intro:off -CPUPriority:high -CPUCount:4

Large Address flag for testing/stability. CPU flags are optional, and may or may not improve performance (another tweaking to test). DO NOT set CPUCount to 1, game will be so slow it's virtually frozen (but not quite).

NOTE/FURTHER INFORMATION: Works BEAUTIFULLY if run via WINE, as in install the windows version of steam via wine and then installing SimCity 4 as usual (resolution tweaks still necessary). Verified with Wine versions >= 5.x. This is not a bad idea, as a fallback option. Some games run better this way than via proton. Also, some games run better with the windows version via WINE than the native linux version.

Command line options, noted above