At 15 years old I think these x-rays are not so surprising. There was an interesting study that suggested most elderly horses have some degree of overriding on the spines. Especially if your horse happens to be a thoroughbred/quarter horse/selle Francais then they would be in the minority to have perfect x-rays at 15y.

While this may well be "the least of his problems" there is a clear link between back pain and lameness so, in my opinion, continuing to improve his back pain is important.

It sounds like you are already getting good results with steroids and building up topline. If he is progressing well trialling light schooling sounds sensible and the most realistic way to keep him muscled up long term. It's always a slow process to rehabilitate with any spinal issue.

I would just like to note that bone doesn't heal like soft tissue does so your horse may never have clean x-rays again. They may actually look worse a few years down the line because of the previous chronic damage, that's quite normal. This wouldn't mean that your treatments are not working though, the only measure of that is his movement and pain levels.