Some toilet cleaners contain hydrochloric acid. It will quickly dissolve calcium. I’m not exactly sure what the problem is that you are having...but I recently had very poor flow/pressure in my shower. I took apart the valve (only possible after using the HCL), and there is a ‘cartridge’ in the mixing valve that was full of calcium deposits. I may have been able to clean it all, but the surefire way to fix it was buy another over-priced cartridge. Its also worthwhile to flush the valve body by having a partner carefully turn the water supply back on (be sure to devise a contraption to make sure the water makes it into the shower/tub, and not behind it). I even fished a tube through the shower head pipe and pumped HCL back down into the mixing valve. Anyway, toilet cleaner with acid will do the trick.
Luckily, I have the time and desire to protect my home and investment in all the money spent on its upkeep!
I have to listen to enough of the, “that part has to be replaced, it’s not made to repair” with my car, that I do not replace every two or three years (actually, years) because when I bought it, I loved it, why replace it just because new ones come out!