It sounds like you have two problems to solve here: temperature sensing and controls.
Since you mentioned radiators, my first question is whether you have one loop for the entire home, or do you have 2 or 3 loops so each zone can be controlled independently? It sounds like there’s one thermostat, one loop, one zone, but I want to make sure I understand. I expect adding in more valves will be difficult, as there’s going to need to be a pair of them for each radiator to either bypass that radiator (radiator closed, bypass open), or go through the radiator (radiator open, bypass closed). My radiant heat system doesn’t have any bypass nor valves, the room-level control is based on whether the vents around the radiators are open or not.
As for the sensor question, as far as I can tell, that will be breaking new ground. There was talk about external wireless sensors a couple times before, and @HestiaPi might have more info, based on their post about adding support for wireless temperature sensors.
I have a different problem that also requires remote temperature sensors. I already have the sensors (Ruuvi tags, which are also an open hardware design and uses open source software), and they transmit over Bluetooth. I do have a collector which jams them into a database, but I don’t want my thermostate to depend on that collector and the database being up. Instead, I plan on building a new HestiaPi with the Raspberry Pi Zero 2, which has bluetooth. Then the HestiPi can see the temperature itself and take action (e.g. turn on the heat) without being dependent on any other systems. So that’s going to be my solution for the temperature sensing side.