Sounds like a crappy design - you should get much longer than 6-months satisfactory use out of it between your maintenance, regardless of your water quality unless it is full of hard debris (sand, dirt, etc.).
quote; It is a thermostatic model,,,no balance spool.
The specifications say it has "antiscald protection" and "crossover protection" so there is something inside it besides the cartridge. But clogged screens might affect the flow to the point that the thermostat would not work properly, but more than likely it is just worn out.
so basically, to remove the cartridge I use a wrench of sorts to remove the cartridge from the photo in Post #1. Could anyone give me any hints on what size/kind of wrench to use?