My back has been sore for a while, and I was always 'rounding' it to try and stretch it out (child's pose, touch my toes, bath with Epsom salt, rolling on my back with my knees tucked into my chest etc)

What really helped relieve tension was doing the opposite, and forcing my butt/hips backwards. Basically, enhancing the dip in my lower back by either using my hips or my shoulders.

Like, standing captain Morgan style (one leg up on stable coffee table, or bathtub) and lifting your arms up and leaning back while also pushing your hips back and stomache out. This should be a gentle stretch! Obviously don't do anything painful and listen to your body, but bending (flexing?) backwards has helped my back where bending forwards just made everything worse.

Try and maintain this shape when bending down, not just stretching, maintaining a straight back when bending over has helped a lot with both keeping tension away and being aware of my back form.

Basically, try to keep the shape in the second image (DON'T LIFT IF YOUR BACK IS SORE). When I stretch like this I'm generally standing straight and not bent over. this pic is just the general shape you want to maintain and 'exaggerate' while stretching.

Also, use ice packs and not heat to relax tight muscles.

It's Kind of hard to explain, but the tension in my back went waaaaay down the next day, and has been significantly better since.

If it helps, I don't go to the gym or anything, by back pain is born of mild sloth lifestyle, bad stretching, and poor form bending over.