My in-laws are in their 80s, but they go 'jam' every week with fellow musicians and I hear you about the light! My best tip is to just to look for those still moments, or at least when they are not wildly swaying, and use a slower shutter speed. I think strumming with slow shutter can be really impactful. Seriously though, this is a tough situation. A wedding photographer would just demand to light the scene. π One thing you might try is a long lens. Getting in close can be magical with musicians. You did great here with what you were given!