Just found my RCT discs again, made ISOs of them so I wouldn't lose them again, and installed through that. (took <1 second 🤣)
The first time I started it, all I got was a black screen with the title music playing. Alt-tab and back again fixed that, but then the main menu buttons didn't work. Close and restart, everything appears to work now.
Except I can't change the resolution. Only 640x480 and windowed mode appear to work. Selecting 800x600 or 1024x768 just switches back to 640x480 again, no error message appears.

Any ideas how to fix this? Or does anyone know where rct stores the resolution setting (so I can edit it manually)?