A manual mixer will work as they aren't restricted as such, But then your chucking money at it.The old shower I think was a plain mixer, it was certainly 12 or so years old and I believe thermostatic is relatively new? Could it make sense to try replacing the thermostatic with a manual mixer then? I suppose it is not guaranteed to fix the issue?
Have a look at salamander give you a rough idea.
SALAMANDER PUMPS
Useally 1 l/min flow per side 2 l/min h/c will activate the pump.