I have had FAI surgery on my right hip (7 years ago) and probably need it on my left, but it is probably too far gone and I will leave it until it gets too bad and go for hip resurfacing now.
The recovery from my FAI surgery was slow. I was riding my upright bike at a fairly high level about 6 months after surgery but with notable discomfort in the right hip. By 12 months I was riding upright at near my highest level (getting PRs on strava) with still some discomfort. The discomfort settled down at about 18 months and right hip feels pretty normal since. At 12 months I was hesitant to call the surgery a success, but by 18 months I was happy.
I also struggle with chronic lower back pain, so got the V20 about 4 years ago for that. I find that my right hip that has had the FAI surgery is fine on the V20. I have not had hip pain issues on the V20 and I ride it at a faily high level. I also continue to ride upright but use the bent to manage the back. I will say that my left hip is generally fine (it has discomfort but managable) on the upright, but on the bent, it feels worse. I am currently in a situation where riding the upright I have to manage back pain, but on the V20 I have to manage left hip pain. So I ride both to do the best job of managing both.
My thoughts are that the bent is harder on the hips, but if your hips have been repaired well and recovered well, you should be fine. I would think the more straight you are at the hips (more reclined), the better for the hips. For me the closer the knees are to the chest, the worse the FAI was. The reason I think the bent is harder on hips is less about hip angle and more about the more constant loading. On an upright you can more your position around a lot (stand, sit, forward/aft on saddle) which kind of moves the load around between muscles which seems to help me to reduce the load on the hips. On the bent, you only have one position (really) and you just keep pushing the same muscles. I live in a hilly area, and can spend 10-15 minutes on a climb where I am just loading up the hips and cannot do anything to change the load.... except put on lower gearing and go slower. On the upright I mix it up quite a bit on a long climb.