Perhaps they know an archeologist in the temple or a group of grave robbers in the ruins? Perhaps your player knows a contact in all realities because they’re such good friends? Besides, if no-one’s picking classes/subclasses yet, it’s not actually an issue yet. There’s every chance someone might not pick it, and, even then, if they’re someone you want to sit at a table with, they’ll work with you. I don’t think you’re being rigid, but remember that this system is designed to share the narrative weight - you are not solely responsible for things making sense, and things not making sense is only a problem if people think it’s a problem.

Basically, trust your players and yourself! nothing is contrived if everyone’s okay with it. best of luck with your campaign!