Why dont you get some seat belt clips, as you adjust them to be slightly looser around you, but still have the safety of the inertia aspect, in case you need to stop abruptly.
Mrs bear and i have them in all our cars and theyre great. Should stop the rubbing on you're titty's.
They range in price and somewhere like Halfords should have too.
You can use an ordinary clothes peg instead of buying a clip. I had major abdominal surgey 5 years ago & found I could n`t bare having the seat belt on my tummy for a long time after. I used a clothes peg just to make the seatbelt a bit slcker. It works . xxx