I started riding at 38.
IAM and RoSPA are, for all intents and purposes, identical. I’ve been a member of the IAM, and am a current member of RoSPA, I’ve been on the local group committees for both, and have been an observer for the IAM and a Tutor for RoSPA. Both work from Roadcraft as the basis of the system of riding.
Key differences will be the organisation and membership of the local group. I found significant differences between groups and how they operate when I relocated. Not better or worse, just different.
I’m not aware of ERS so can’t comment.
EDIT: Ah, the Enhanced Rider Scheme (Google was my friend).. it seems this is really an introduction to advanced riding. I was encouraged into the local IAM group from a Bikesafe course... anything that makes you more competent and safer has got to be useful.
I would recommend doing something to extend your ability as an ongoing exercise. Track training days (not track days*, but days where you’re taught cornering technique), trail riding, i2i courses... even Motogymkhana... it all makes you a better rider.
*though they can be fun ... with the right organisation.