Well, it's beginning to look like the path of least resistance will be to use the old bedroom that was going to be the master closet and make it the master bathroom instead. Instead of trying to maneuver, relocate, the stack vent, tie in two bathrooms into it and trying to put 4" drain in a basement with HVAC ducts galore, just isn't easy.
Instead I can run the "soon to be" 1" hot and cold line over and build a new drain system to exit out foundation wall right into the septic line.
Removing the santee and replacing with a coupling is so much easier!
All I will need to focus on is getting hall bathroom up to code. Will need to involve a roofer and a plumber to do the new soil stack vent. Yes, I will have the soil stack vent right behind the toilet!