The ell of our house with kitchen, pantry and dining room has radiant floor heat. It has its own thermostat and several loops feeding off one manifold. We don't change the heat there over the short term. We might turn it down if we will be gone for a week, but we don't change it over the short term.

In our baseboard heat in the rest of the house , the radiators change the heat fairly quickly since the water directly heats the air, but the radiant floor heat first has to raise the temperature of the floor and then the floor raises the temperature of the air above it, so it's a much slower process. I don't find that the rooms heat more than a degree above or below the set temperature.

With 3 windows and high ceilings, you have a large air mass with heat loss through the windows, so your time to heat the bathroom from a lower temperature will always be long. To my mind, it makes the most sense to keep the bathroom temperature at whatever heat you like and leave it there. It's the nature of radiant floor heat to be easier to maintain temperature than to quickly change temperature.

You want a thermostat that is specifically designed for radiant floor heat and will minimize overshoot. Something like a Wirsbo/Uponor or Tekmar.

There's no problem with mixing different types of heat in different zones on the same boiler, as long as it's set up correctly.