You know, I've been in your shoes twice. Injured, had surgery, injured again, another surgery. Both times with the supraspinatus.
I will honestly say after the 2nd surgery, my shoulder didn't feel right at all. Surely on an anatomical level, the damages were repaired but it just doesn't didnt feel right. I had nagging pain and constant soreness each time served.
I kept at it, I kept working out, doing the rehab exercises as instructed by my PT. You'll be amazed by work is needed to strengthen a rotator cuff.
I could hit groundstrokes all day long, but I couldn't serve out an entire set without pain and soreness. I realized no rehab exercise would ever replicate the motion and stress that a shoulder goes through during a service motion. I call this "serve stamina", so I worked at it.
Started off with 50 balls nonstop, then 75 balls nonstop, 100 balls, so on and so on until I could serve anywhere from 200-300 nonstop and not be hurting.
After that, I worked on my motion. I could have the strongest shoulder in the world, but if my technique is flawed, then no amount of rehab will preserve my shoulder. I have since made slight changes in my original motion to take away as much of my shoulder as I can, while incorporating more legs and core while increasing power and spin on my serves.
In the end, if you're determined and willing to put in the work, the human body can come back from any non-disabling injury.
Tips for you based on my own experiences:
1) Rest until you're not in pain.
2) Work and rehab the injury. Follow the PT down to the T. If some fool at the gym shows you some random shoulder exercise that makes you raise an eyebrow or two, chances are your gut instinct is right.
3) Court stamina/serve stamina is huge I think. Muscle memory accounts for a lot of the strength/flexibility.
4) Think about your technique, if something's hurting then chances are, you're over-stressing that particular body part.
I also advise against the cortisone shot. Surely it'll relieve you of the pain, but it turns off the inflammation that starts and facilitate the healing process!