I mean the portal and it's defenses seen like something that would already be prepared in a campaign that involves traveling to the plane of fire. Things like divination and knowledge dcs would likely have to be improvised on the spot (ie DM fiat) or over prepared in advance (also dm fiat) unless the dm is specifically catering to pc abilities when designing the campaign (very much DM fiat). I think the dm goes much more or of their way to accommodate the players with plane shift spell than those without.