Uptempo. I've had more injuries than anyone! Your description of 'tense', 'poor posture' etc. describes me exactly. I can play for two to three months, then I have to lay off for 8 months whilst I recover. The last episode (numb hands) started early January, I didn't feel fit to do anything (let alone guitar playing) for some 5 months. Any activity that I tried seemed to make matters worse. I've had electric shocks (obviously nerve problem), concrete shoulders, wrist pain, hand pain. Thankfully not all at once! Over a couple of decades though. Lengthy bouts of not playing at all, in some cases for years.
Try Alexander (I haven't). You should also do some type of yoga exercise but I can't possibly say whether you should do these in your current physical being. You may have to wait until your condition improves or go to see a specialist and ask them if it's OK to do those type of exercises.
A real common problem for guitarists is the shoulders. We tend to 'fold ourselves' around the instrument, shoulders push forward. Once you start doing that it becomes the 'normal' position. In fact anything else will feel wrong to you. That's what they term a bad habit and can lead to all manner of problems. Practicing doesn't make you better it makes you worse, as the tension becomes more severe. It's a cycle that I've been through many times.
The one exercise that really does seem to be helping me is 'wall angels'. I knew about the exercise several years ago but being a lazy t*** I rarely did them. Now on the advice of a physio I do them several times per day. They were extremely difficult for me to do, still are. I cannot really get my hands fully back to the wall. Things are certainly taking a turn for the better but it's going to take much longer to undo years of poor posture.
I do both of these exercises.