tbh i may suspect the shyte system water isn't helping either but the limescale produces the effect also , also problems may exist with the shower/ cartridge.
this is a picture of a plate being affected by limescale alone which i see a lot where i live which reduces the hw flow also but descaling this plate is the way to go
View attachment 133287
Hi mate how much do you charge for something like that?
I wish you were local lol