My experience is lots of unsuccessful troubleshooting. I will venture to guess that "not going to zero vac" is not what is preventing it from going into fourth. Zero vac, which can be considered max vac release from the plastic valve on top of the IP, happens at full throttle, or at the minimum, 3/4 throttle. So what if the car is driven at half throttle until it reaches 40-55mph? It should shift into fourth.
As far as swelling the seals. Lucas trans fluid works great. It smells and looks like atf but has the viscosity of 90w. I had a sd that would never go into fourth once it was warmed (=thinner fluid), just spun as if in neutral. It went into fourth with 2-3 quarts of lucas trans fluid in place of atf. I had nothing to lose, the trans was basically shot. I would use less than 2-3 quarts if I was worried about ruining a good trans. Ended up buying a new trans and it was very disappointing since I didn't have correct vacuum it didn't shift nice. It was improved in bits and pieces but never perfected and then the car was sold. Thought I was going to hold out for a manual trans car after that. It was a combination of thinking that I can somehow now fix vacuum issues and that manuals are so hard to find (or too expensive), bought another auto. No test drive prior to commiting to buy. Hard 1-2 and goes right to 3rd. I hope this isn't causing damage. Once the trans breaks, it will likely be parked until a manual conversion is done. If I had complete control of how it shifted mabee a new automatic would be an option.

1984 300d (210k +/-)