Going back to 1e AD&D, the MM entry for type VI demons notes that "[e]ach type VI demon has its own name. (Balor is a type VI demon of the largest size.) Six are known to exist." which tracks with the entry in the Hype3e Referee's Manual for both type VI greater daemons and Thaumagorga. The DMG appendix E then lists out the following names under the entry for type VI demons: Alzoll, Balor, Errtu, Ndulu, Ter-soth, and Wendonai.

Of course, I have a fondness for the classic named fiends, Orcus and Anthraxus and the like, though choosing six and Hyperborea-izing them could be tricky and some of the big ones feel like they'd fit in elsewhere (Demogorgon feels like something to do with Xathoqqua to me, and Juiblex might be an appendage of Yog) so probably not saving oneself too much time going that route. Still, raiding an ancient Hyperborean temple to Orcus, general of Thaumagorga sounds pretty good. Could even keep the first option as like, Thaumagorga's praetorian guard or whatever, in case this fractious set of princes steps out of line.