Most shoulder pain falls under the umbrella of Rotator Cuff Related Tendinopathy and/or Subacromial Pain Syndrome. Please stop calling it “Shoulder Impingement”.
Subacromial decompression does not really work for subacromial shoulder pain. If the acromion was the culprit, you would expect this surgery to improve function and/or pain. The culprit is not the acromion.
The issue is probably intrinsic to the rotator cuff itself. For the deconditioned/sedentary, they need more load to build capacity. For those who resistance train regularly, load is likely exceeding capacity; ideal management consists of exercise modification and/or adjustment of volume, intensity, frequency.
Tendon pain warms up to exercise. I wouldn’t worry too much about pain during or shortly after training. Monitor symptoms from week to week and/or month to month.
Stem cells, PRP, and corticosteroids do not really help for tendinopathies. In fact, they can make things worse. Corticosteroids are chondrotoxic. With regards to PRP, see below:
"PRP injections within interstitial supraspinatus tears did not improve tendon healing or clinical scores compared with saline injections and were associated with more adverse events (pain >48 hours, frozen shoulder, extension of lesion)."