Is the issue new? Hold old is your seat?
I had similar issues with my road bike seat. I tried the adjustment of seat angle - didn’t work (back would get arched in the high position or I’d slide forward on the lower position; no “just right”). Then I tried a new saddle - didn’t solve the problem. Finally, I happen to be test riding a new bike with a new version of my current saddle and that solved the problem!
FWIW: The seat I have been using is the Specialized Toupe. When I called Specialized, they told me they last a very long time. When I talked to other serious riders, they change them every 2-3 years.
FWIW2: I have a Specialized Hinge (model name) on my MTB and love that as well - sort of the Toupe version for MTB.
btw: I have a very strong core due to a robust strength training program I have been doing for a couple of years. I’d highly recommend strength training overall, and core as part of that if you aren’t doing it already.