Warning: This will be heavily biased towards my specific setup. If you have had issues with boosterless brakes, this might work well for you though.
My brakes finally feel ok. I just finished a drift event 2 days ago and forgot that I even had issues with my brakes until half way through the day. They do take more effort than factory but not much. I feel like the firmness of the pedal is what spec E30 guys want in their cars.
The solution to my issue is still not completely clear. There was a spec E30 forum post I read where a driver hooked up pressure sensors to his brakes and depending on different inputs, the rear brakes would either work or not. He tracked it down to the rear proportioning valve. I debating on just deleting it but instead I replaced it with a factory unit for 12x the cost. At the same time, I swapped to a different (but used) factory cylinder, and deleted the ABS unit. The brake fluid from the old setup was pretty dark so I opted to replace the front brake lines as they were the most worn.
With all of those changes, my brakes felt great. In retrospect, I don’t think I know exactly what did it. My last master cylinder could have been internally fried from the heat from the headers. As a precaution, my new(er) master has a heat shield that attaches to the master cylinder mount. It should be attached to the engine but space doesn’t allow for it. Regardless, I could still grab the master after pulling off track from a run.
The other options are either the ABS delete or the rear proportioning valve. I kind of doubt the ABS unit because I couldn’t find it failing for anyone else. The online spec E30 forum post makes me think it was the rear proportioning valve more than anything.
I highly doubt swapping out the lines and bleeding the brakes did anything but you never know. In the end, this manual braking setup with the moved pivot point will work with the factory master. I went down the rabbit hole instead of simply investigating the rest of a system I didn’t know.