You're right that ORMs aren't really set up to use jsonb but it can be done. I've had pretty good success with a kind of "hybrid" putting most relational data in regular columns (with FK constraints) and adding flexible data in jsonb. The main trick is understanding how to build custom column types in whatever ORM you actually use.
I can imagine a future with specialized ORMs designed around jsonb, but the current state of the art is probably not as bad as you think.