It's pretty clear how many wheely bags will fit onto a certain aircraft type in the Y and J cabins. Here's an idea:

Have two boarding times. Board Y first (in priority order), counting the number of overhead bags boarded until limit is reached. After wheely bag limit is reached, check all further bags unless they can fit under the seat. Keep J lockers closed during Y boarding, enforced by crew.

Boarding for J starts 10-15 mins after Y allowing extra time in lounge/later arrival at the airport. Also allows time for any extra Y bags to be put in the hold. J bags should always fit comfortably above the passengers' heads and there is plenty of underseat space below B and E. Crew can move bags around in Y if necessary to make room for an extra J bags. Guarantee all J pax will get the full allowance (i.e. one regulation size wheely and one small bag) in the cabin on every flight, even if this involves removing a Y passenger's bag and putting it in the hold.

This creates actual value for J, means that J pax aren't sat there unnecessarily whilst everyone boards past them, and reduces the pressue of getting your bag in the locker early for J pax.