The main muscles controlling the hands are, apparently, actually in your arms... hence, when dealing with handwriting issues, you have to think a bit further than just the hands (and also accept that if you have overly stretchy tendons and ligaments, your hands will always be a bit difficult to stabilise for writing, because these are what are being pulled on by the muscles to control your hands). Physio, therefore, can be extremely helpful, as they understand how the muscles in the body are supposed to work and can give advice and exercises to build up strength holistically. To have good handwriting, you really need good core body strength, stable shoulders and hip girdle, etc, etc.... OTs can also help with advising on aids, eg, pencil grips, writing slopes, different types of pencil or pen, games and activities which can help.
My ds1 is hypermobile and the OT advice for him was rubbish. However, he had an excellent physio. What made the most difference for him, though, when it came to writing, was learning to play the piano...