I don’t play Poland, so I may be missing the point, but they seem decently powerful to me. Not OP, just solid.
Civs that get religious bonuses (not just faith bonuses) always get at least an indirect bonus to getting a religion, because their bonuses make investing in a great prophet more worthwhile and therefore better value. Poland gets three boosts here: the culture bomb and relics thing; the wild card slot (which directly makes it easier to get a religion); and the district bonus for Holy Sites, which makes Holy Sites more valuable (making them a better investment).
To me, their HS adjacency and Unique Market just scream city centre commercial hub holy site triangles, maybe putting down an encampment or fort at the edge of disputed borders just to mess with opponents.
All that extra faith - from Holy Sites, Relics and being able to run religious cards in their wild card slots - we’ll, I would have thought that would significantly boost their strength for religious victories. I’d have thought they’d also be good for faith / religion based dom, particularly using defender of the faith and their conversion ability.