Sorry to hear about your troubles, Dan.
I'm about 3 weeks post a partial knee replacement. I've also used the bike a fair bit as rehab, but what's really helped, is gradually lowering the seat to work at the bend degrees. I'm not sure if that's something you've considered, or if it's too late, but a thought, anyway.
Also, I've found a great deal of the remaining limiting factor for bend is the amount of swelling that remains. Are you having any issue there that might be contributing? I've found a good ice and elevation session prior to the bike/next stage of seat lowering helps get that bend greater to where I can work on the scar tissue.
best regards