There’s some good advice on here Steve, both on improving your seating and keeping fit.
I’ve experienced a lot of lower back pain in the last few years and I would recommend finding a good physio who specialises in lower back/sciatica problems.
Mine uses a little acupuncture to ease the muscles while manipulating and then giving me stretching excercises to improve core strength.
Most of these excercises are in fact Pilates moves and she suggested trying Pilates classes which I’ve found very helpful.
I’m 69 and a regular gym/swim goer plus the Pilates classes.
Hope that helps
Good luck, keep fit!
Peter