The solution’s pretty heavy duty and not possible for everyone, but I finally fixed this ridiculous problem.
Deleted my account from Xcode 11.something and then uninstalled it (apparently it’s known to have issues with macOS Big Sur, such as randomly not caring about your certificates bc your login session is voided, and it’s physically impossible to restore it) (also deleted all residue hidden in /Library), revoked all my certificates, voided provisions etc. Downloaded a fresh copy of Xcode 12.something, generated fresh set of certificates and provisions, disable automatic signing in UE4, and compile. Everything’s working again :- )