many hot water bypass systems only have 1 quarter turn valve along with a one way check valve on the other port. The one way check valve tends to have a very non linear restriction vs. water flow and pressure. So.... as you turn down the flow on the shower head, the restriction on the one way check valve goes way up and you get an imbalance in hot water flow, much less than expected.
Solution, toss the check valve and install a bypass system with two quarter turn valve. Problem solved.
And if you want to really improve your shower control, install this
shower valveand then keep the oxygenetics head, but no use the valve to control flow and get constant temperature.