I’m going to throw a whole bunch of thoughts at you, so pick and choose what makes sense.
I"ve had times when my back bothered me every time I rode and for a while afterward. When I went a few days or even a week without riding, it felt a LOT better. So, I looked hard at my horse and her posture, my posture on and off the horse, my tack**, my level of tension indoors/outdoors, even my boots! And every single one of those things was a factor.
One solution that worked for me was to make a commitment to deconstruct my seat. My horse has some soundness and consequently, posture issues (she loves that head up, hollow back way of going and if she gets going forward enough to use her core correctly, her stifle bothers her). We started with a Connected Riding instructor who showed me ground work I could do with her, and mounted work I could do for both of us, and it has made a world of difference.
In the course of this, I realized my saddle was contributing to the problem so I"ve mostly ridden her bareback for the past year. Now I need to do some serious searching for a saddle that helps us both sit and move correctly. Need that winning lottery ticket!
You asked about stuff you could do mounted/unmounted while you’re taking it easy on your back. If you have someone who does any of the Tellington-Jones groundwork, it’s fun and complicated enough to be challenging and not boring, and it’s good for your horse and you. There are books to show the basics. Don’t worry about the funny names for things, try to see how the horse is standing and moving and give it a try.
Oh, and the boots – tight hamstrings will make your lower back miserable, and some boots contribute to tight hamstrings. I spent 3 months going to spine class at one time, and hamstrings and quads were as much of the workout as traditional core work. Just like our horses, we learn to compensate and then end up with even more problems.