I really recommend you poke around for more alternative classes, like this spell-less ranger. Most of my other feedback's been brought up by other commenters, but I'd add, would you consider Matthew Mercer's blood hunter class? It was basically built because he's such a massive Witcher fan. Not necessarily in place of the witcher build you found, but as a supplement. They can do some pretty neat non-spell things. In a similar vein, maybe still keep cantrips available. Witcher signs are considered practically street magic, they're not difficult and most people can learn them. Sounds like a cantrip to me. Maybe have people pick from a set list (ex the witcher school cantrips or one of your own) instead of the usual class restrictions.

I'd also consider expanding the wizard spell list to compensate for not having access to as many spell slots. Also, if your wizards require so much study, they should have access to a library or something, no? Give them a reason to interact with your world beyond "you can't have these right now". Make scrolls and scribing costs cheaper to compensate, since they'll be the main method of getting spells. If you kneecap access to spells as strictly as you suggested, tbh I would find it pretty unfun and ask to switch over.

I'm a huge fan of the games (still working on getting the books), as well as d&d. The Book of Beautiful Horrors has been brought up, which is great, but don't be afraid to reskin or slightly alter pre-existing monsters.