Maybe I just put a TRV on the "always on" rad given the whole system is going to be always on.
Not that I'm an expert on these things at all, but when I asked why that existed (the always on radiator), I understood it was that if all radiators had TRVs, on an old boiler (without a separate bypass circuit), you would be stopping circulation and the pump would blow.
Even with a modern boiler with a bypass circuit, you'll still be running the pump and wasting energy anyway. And a TRV costs almost as much as a cheap thermostat, and fitting it will probably mean draining the heating system, compared to connecting 4 wires to the boiler.
Essentially, your original question is 'how do I efficiently and simply do what a thermostat does'. And the answer is 'fit a thermostat'.