My mare has been stiff ever since she went backwards over a fence (luckily I wasn't on her at the time) so she has visits from the back lady so I ticked chiro but quiate a bit has been done by the vet too.
Ours all get stiff being multiples ponies. They are all done before the summer season and it really helps, they probably would benefit from more. I'm pretty sure the lady we use is pretty much qualified as all of your options, but we call her a physio.