Related experience for me recently. I've done planks / side planks / reverse planks three times a week for years now, but still get the occasional lower back pain. Every time I've tried to start runingn over the past couple of years, I've ended up with back pain that would get gradually worse until I stopped trying. Like just short jogs even.

Doctor did x-rays, had me do hamstring / hip flexor / lower back stretches, sent me to a physical therapist who had me do the same, with no luck. Recently I tried getting back into running again thanks to being home more than usual, and was surprised to find that I didn't have any back pain like usual. I thought it might have been because I've been working on strengthening my legs more. Seemingly unrelated, I've also been playing a ton of beat saber thanks to the extra free time.

I was getting some back pain over the weekend, and it got a fair bit worse after my run on Monday. I noticed that I had stopped playing Beat Saber for a week, and played for half an hour to at least be standing up....and my back pain went away entirely. I've played a bit of Beat Saber every day again since then, ran two more times, and haven't had a lick of back pain. Not advertising Beat Saber in particular, but I think having an activity to stretch out your back regularly and keep you from sitting from a while makes a world of difference in a way that just stretches can't.

Also back when I was getting the back pain, I found that crossing my legs would make it way worse. If you keep your feet flat on the floor at all times while sitting then it seems to make a lot of difference too.