Hi Eddzzz,

I spoke with few people who had prolotherapy treatment and it can really help to strengthen ligaments around SC joint but important is the technique this treatment is applied. It should be done using the microperforation technique under radiographic imaging. It's nice to join prolotherapy with PRL (platelet rich plasma). I already had PRL when I had Achilles tendon inflammation and it helped. I will try to get that treatment at some point.

For now I found really helpful video how to self massage subclavicle muscle which allegedly is overworked for most swimmers plus I overworked my arm by pulling so this muscle worked hard as well. It sounds crazy but before massage something skipped around my clavicle every time I shrugged my shoulders or tried to do circles with them. Now is all right. The same after whole night when I tried to raised from bed there was loud crack.

Because you are swimmer too if you want just have a look. Who knows maybe it will be helpful to you as well. Or maybe you know about this already. Nothing too lose (just a couple of minutes). Please find link to self-massage of subclavicle muscle video.

Self-myofascial release: Subclavius - YouTube

and the same massage but a bit advanced method.
Subclavius pain & trigger points - myofascial release - YouTube

Let me know what you think.
All the best Agnes