The bolts use a plastic washer and plastic nuts that slide into plastic rails of the plastic cistern. It’s really flimsy. As I tighten the nuts, I can hear the cracking sound of the plastic washers cracking internally. I’ve reinforced them with a bit of thin sheet metal but whatever I do cannot rely on bolts to add much pressure. This is also why I kind of doubt a rubber flanged gasket working even if I could track one down.
But indeed using all gummy stuff would likely work (but messy!). I’ll try it if my next plan fails.
This is precisely what a thermostatic valve gives you. I don’t think you know how a thermostatic valve works. If the thermostat on the mixer bar is set at 38°C and you turn on the pressure control when the water in the hot line is cold, a thermostatic valve opens the hot 100% immediately and when the hot water arrives at the fixture it automatically adjusts. What you say about starting 70% open is exactly the problem you have when /not/ using a thermostatic valve, like the pressure balance valves that are common in the US.
(edit) also consider how tankless heaters have a minimum threshold for turning on. If you open the hot valve just a little, the heater will not be triggered. It must be open enough to trip the flow switch. It can be tricky to open the valve just enough to trigger the heat. A thermostatic mixer valve solves that.
I don’t think you saw the pic I posted of the cheap thermostatic mixer bar. Those can be installed by anyone because they are external. You just turn two union nuts until hand-tight then give ¾ turn extra with a wrench.