One of my brothers swears that a few short yoga sessions at home, after rides, has helped him tremendously. He piles up the miles annually. I don’t like stretching, and I get a little back fatigue as the OP described. I usually stand and stretch, twist, continue on. For me I think it’s due to prolonged efforts without that muscle group moving much, but in constant use/tension. Short relief breaks help. On hillier rides where I stand a lot more regularly I don’t get back cramps at all, so for me, I think I have it figured out.