Now that's a good idea! I didn't even think about it, just adding sorbitol as plasticizer, but indeed less hardening could be good. More hardening improved the results with unsubbed PET as it reduced swelling that finally loosened the emulsion as its size changed compared to base. But, indeed, less hardening could improve even the adhesion because it gets more flexible just like the acetate base is much more flexible than PET. The optimum might be somewhere in halfway.
I also suppose that 30 um is quite a thick emulsion for a BW film. Right? Thinner emulsion would also reduce curling, which is a major problem now. Scanning is impossible with most scanners and their lousy holders now.