How do I equalize the oscillating cold and hot water pressure in shower?

Background

Hi there. Newbie here from South Africa.
A couple of years back we moved into our first home. We have this problem where the water in our shower keeps oscillating between too hot and too cold.
Adjusting the water requires very slight adjustments on the taps to correct. The correction is delayed and overshooting is common. The hot water tap needs to be opened up a lot and the cold water tap only needs slight cranking.
The geyser is a low pressure keyser with a pressure regulating valve (Latco Valve) on the input of the geyser only.
The cold water is feed directly from the water main line.

Question:

What can I do to fix this oscillation?

What I have tried:

From Google I have seen a drawing where the "Latco" low pressure regulating valve is installed before the geyser input AND before the cold water that goes to the showers and rest of the systems in the house.
I am however not sure if this is a good or correct way to install the pressure regulating valve. All the previous installations I have seen the valve was only installed on the geyser input.

Any help will be highly appreciated.