Ummm dont play a system and limit it when there is a system for the game you want to play.
The Witcher RPG (Book) https://www.amazon.com/dp/B07JJWJTF8/ref=cm_sw_r_cp_api_glt_fabc_JD4R9F1D7XNZ5XA5C444
If you cant afford that system there are other low magic systems that will work better that simply dont have those races or that broken of a magic system.
Have you looked through the actual Witcher RPG Corebook or Expansion or the Original Books for inspiration?
The Witcher RPG Core Rulebook https://www.amazon.com/dp/B07JJWJTF8/ref=cm_sw_r_cp_api_glc_fabc_XyoZFbMCEWNPJ?_encoding=UTF8&psc=1