Offer it as a feature only available if they pay $(3-5x estimated costs).

If they bite, wahooooo!

which makes the cheaper approach by filling a second database by events and providing access even better, just want to check if i didn’t miss any simple option as for sure these kind of requests are quite common in b2b

Postgres offers some views and filters by account or something if I recall, but I don't know anything about it (and I'd still be skeeved out by giving someone raw database access when the system hadn't been designed from the ground up for it).

https://www.postgresql.org/docs/current/ddl-rowsecurity.html

Indeed this looks a lot like a custom request