I suffer from lower back pain often. I have found the best way to combat it is to do a psoas muscle release a couple times a day. You can see the exercise about halfway down this page:
https://www.spinningbabies.com/more-info/for-pregnancy/daily-activities
I also sleep with a body pillow.. bottom leg straight out and top leg leaning over the pillow (agh, horrible description) so that I am leaning over the pillow rather than directly on my hips. I still switch sides each time I wake up, but rarely wake up in the excruciating pain I was toward the end of second tri.
I also want to ask how supportive your mattress is? We ended up buying a firm memory foam mattress in January and this made a HUGE difference because my body was no longer having to fight to keep from rolling down the middle of our old crappy, weak mattress.
AHA! Found a picture of the sleep position I suggested... first picture, top left.
https://www.spinningbabies.com/techniques/activities-for-fetal-positioning/posture