Using private DNS on a VPN defeats the purpose. You want to blend in with the other users you are sharing a VPN IP with, so I would stick to VPN DNS.
Also, don't know what your threat model is, but most here won't touch TikTok or Facebook for privacy reasons. As someone suggested, I'd put those on a secondary user profile and make your main user profile a privacy profile. JMO. You do what makes you happy.