Update on yoga: I've persevered with the yoga class. It's still a struggle to get on the floor and back up again but there's something working. The prolonged stretches and breathing do seem to have made a difference. I'm aware that my upper body flexibility is really pretty good. I have fused vertebrae in my lower back though which doesn't help. However, my ROM is very noticeably greater. I didn't have any complaints about it before but it must have increased a lot. Any residual stiffness has completely gone too.
I can't say I find the class easy and especially enjoy the relaxation session at the end but it's certainly been worth it for me.