Stating the obvious -
Your issue is that there's a significant difference in pressures of the hot and cold supplies to the shower. This means that the 'sensitivity' of the two valves i.e. the change in flow for a given angle of opening is different, so that makes it difficult to control.
Using a monoblock mixer instead of separate taps in this situation doesn't help much - you end up with the control almost all the way to the 'hot' end and it's still very sensitive.
Take the cold supply from the same supply as the supply to the geyser - this is the solution you found by search. This does work.
Add a second pressure regulator to the cold supply so that it holds it down to the same pressure as the one feeding the geyser.
Thermostatic mixers have a bimetallic element built into the valve that automatically shifts the valve to keep the outlet temperature constant. The control then sets the desired temperature, not the mix directly. I've found that these are also a bit slow to respond to changes in pressure, say from someone flushing a toilet, though they will still have better control over temperature than a regular mixer in this situation.
Pressure balance mixers have a spool valve (there are other kinds, but this is the type I've used) which can move to partially block off either supply to the mixer if the pressure falls on the other supply. There's then a regular mixer valve as part of the same valve body. I've found that these work very well - it's the pressure that can change rapidly, not the temperature of the water, and in your case would serve to equalize the supply pressures.
And finally, my solution, since I live alone and there's no-one else to complain about it... I set the thermostat on the water heater so that it's comfortable for the shower with no addition of cold water. It's still hot enough for the washing machine - I set that to 'hot wash' where I'd normally use 'warm wash'. Dishwasher has its own heater. The downside on systems that have a hot water tank is that the amount of heat stored in the tank is less, you use more of the contents of the tank per shower, so it'll run out sooner if multiple showers are taken in succession. The upside is that less heat is lost while there's no usage.