This is not a surgery you get for "prevention" purposes and I am very much of the mind usually "rather do it now than later." I had two shoulder surgeries previously. The first one was for my left shoulder that was absolutely trashed. A year later I did the right because it was gonna be in the same condition eventually, but had that been my only shoulder affected, I wouldn't have been at a surgery level yet.
In regards to a first rib resection, so much can go wrong leaving you worse off that I would not recommend it as a preventitive especially if you're getting better with PT. If you're debating surgery, it's not the right time to get it. 9 months is not a long time to have TOS and to be working on it.
You never know what will happen in the future. Perhaps you'll need the surgery later. But if you can avoid it at all, I recommend doing that. Nerve damage, clotting, permanent debilitation, collapsed lungs... all complications of surgery and the rate of success is not very high in regards to other surgeries. You're looking at a 70% success rate where success is measured by "not worse than it was." The percentage of people who actually feel better after surgery especially with NTOS is lower.
Now, you didn't mention what kind of TOS you have. If you have something like arterial TOS and your doctor is recommending surgery, that may be a different story. That is only 2% of people with ATOS, however. That can be dangerous to not have surgery for.
2 months of PT is a short period of PT. From what I understand, surgeons usually don't want to even think of surgery until you've been in PT for at least 3 months. Did you have prior PT? If not, I really recommend postponing surgery if you're seeing results in what is considered a normal time period for TOS...