Disclaimer: my main reference is me, so I have no firm idea of how applicable it’ll be to you.
I’ve ”always” had less-than-average mobility in my lower back. And a L5 disc rupture certainly didn’t help.
I’ve done yoga and targeted stretches for YEARS. And while that has made me more comfortable WITHIN my range of motion, it really hasn’t added much. Basically, I get more use of the limited range I has, but I can’t bend, stretch or reach any more than when I started.
It took me some seasons of aches and frustration before I realized that the average fitting rules may not apply to someone who hasn’t got average range of motion or basic posture.
Now I ride a frame that’s supposedly too small, have the bars level with the saddle, and I ”never” use the drops.
That’s good enough to manage full-day rides with no more than tolerable aches and pains, and I’m content to settle for that.
What has helped with upper back/neck issues is swimming(crawl) and the pole work that comes with Nordic Skiing.