Sounds like the pressure balancing module is bad.
Sometimes it is built into the shower cartridge.
However it is often a separate cartridge or module, sitting behind the cartridge, further inside the valve body.
And what is that second valve on top, does that divert water to body sprays or a hand shower or something like that?
That top hand shower diverter likely has no effect on hot water output.
That red ring on the lower main valve looks to be a limiter, it has a bump on it that stops the shower valve handle from turning all the way to full hot, usually as a safety to avoid scalding.
I don’t know the brand of valve, but it looks like you can unscrew the brass cap on the main valve by gripping those two flat spots with a big Crescent type wrench, or some really big Channel lock pliers.
To access that red limiter ring.
With the house water off first, of course.
And open the valve to let shower water pressure out, and open a faucet lower down in the house or nearer the meter, to let some water drain out first.
That way less water bubbles out of the valve and less water gets into the wall behind the valve during this procedure.
That red ring can likely be inserted in several positions, to allow more or less movement of the valve handle towards the full hot position.