Hi Helen - I did tailoring for a long time as a side thing. Many people with their "native shoulders" (my ortho told me that really is a term!) have uneven shoulders - sometimes by a lot.

My solution was to make custom shoulder pads to make up 2/3 to 3/4 of the difference. For women, I would include a loop & snap to attach to the bra strap. For men, a bit of non-slip fabric on the underside. If made of a skin-toned fabric & feathered out at the edges, these are invisible under all but the sheerest of tops (or sleeveless, of course.) Another option is to remove the sleeve, lower the shoulder cap, taper the sleeve and reattach - a lot more work.

If you look in a sewing supply site like this you can find many options:
https://www.wawak.com/garment-construction/shoulder-various-pads/
Have you talked to a Rehab Physical Therapist (one who evaluates and designs custom programs for patients, not a "big box" PT company) about designing exercises to help build up the deltoids and other muscles?