[2023] What causes special zombies to spawn?

Rad zombies aren't really a 'special' zombie. They're a higher difficult variant of the basic zombies. Basic->Feral->Irradiated.

Late game will essentially be mostly irradiated and bombers. But they stop scaling after awhile, while you can build your base better and continue to get better gear. If you can find the rad remover mod you can equip it to a weapon to nullify irradiated zombie HP regen.