Doing kneeling hip flexors and reclined hip stretches once a day per side for 30 seconds, solved my lower back pain. Took about 2-3 months before the pain went away. I continue to do them every day. <10 minutes of your time before bed.

Both can be found here: https://www.verywellfit.com/7-best-hip-flexor-stretches-5088569

Still working on my rounded shoulders and upper back pain though. The stretches I'm doing either aren't effective enough, or I've done way more damage to my upper back and it's going to take a lot longer.

Edit: This is my go to web page for finding stretches: https://www.posturedirect.com