How do I fix low framerate? Love to play for nostalgia sake but it's simply unplayable.

type on console /r_primitives 2 and /com_maxfps 125

If it works, /seta cmd value to keep it saved

works smoothly now :) thanks. How do I prevent the game from crashing when setting a higher resolution? (OpenGL error)

Avoid setting an impossible resolution ^^'
If too late, edit your etconfig.cfg, set r_mode to 4 (default, 1024x768)

Ah I thought highest possible was an available option but on closer inspection I can see it's indeed outside of my native resolution's maximum :')

For a better native, modern experience, you can try ETLegacy
https://steamcommunity.com/sharedfiles/filedetails/?id=2800452649