Renovation of heating system for existing property - condensing boiler/air-water heat pump/DHW heat pump

If it is not a problem, and it probably isn't, that the bedroom is one to two degrees colder than the floor below, then that fits perfectly. Then the radiator can also be nicely placed in the supply line of a small heating circuit, and no efficiency-draining mixing is required.

It's like with so many things: once you understand the rules (and reasons), you can occasionally break them. ;-)

Edit: Additional idea: Install an air conditioner in the room. It helps to sleep well in summer, also cools the floor below somewhat through open doors, and can also provide heating upstairs in an emergency.