I am one year out of back surgery (disk fusion) and wondered if I would ride again after the surgery. The thought of sitting upright while bouncing over road bumps sent chills up my repaired spine. But- I just finished my first lengthy road trip last week (180 miles) and found a few key things;
1) A back brace made the trip much, much easier. I used the one I had after the surgery, but there are many out there you can get to support your back. I could even hide mine with my vest! I recomment that step first and foremost.
2) There have been some discussions in here about back rests. That would be a great thing to consider so you could lean back a bit and not put direct pressure vertically on the back.
3) There is a lot to be said about a comfortable seat- both butt and back-wise. Though they are expensive, it is money well spent- especially if you have other potential aches and pains.
4) In consideration of another bike...It's no secret that our 750's have as much power as the bigger bikes, but the realization is that the power comes in a lighter package. That means if you get her in an off-balance lean, you are less likely to do further damage than you would a larger, heaver machine while trying to save your bike.
5) And lastly- you get another bike (or quit riding altogether), you lose contact with this great group on this forum!!
Good luck and let us know what you decide in the end!