is it possible to fuck the brunet
Depends on your definition of "best". I've done all her routes, but the full dark route with her is...extreme? I also am not a fan of subjecting people to unnecessary BS. IMHO, go either Lust or Purity route...with everyone. Or just pick whatever the hell you want. I have multiple playthroughs with full Purity, Lust, and Darkness; as well as a "mixed".
Zoey Raven does walkthroughs for many games on their Patreon, and has one for Radiant, though only to end of chapter 3. Check the master list. And it's free.