How do you use the value from another entity?

I have a Shelly TRV and a Shelly Motion 2 in every room in my home. From these, Home Assistant knows when there is someone in the room and what temperature that room is at.

Next, I would like to be able to set the target temperature for each room. Before I get all excited and purchase kit, I want to try it in software first, so I have created a number helper called “Bedroom Target Temperature”. Unfortunately, I cannot see how to set this condition;

IF [bedroom temperature] < [bedroom target temperature]
THEN [radiator valve postion] = 100
ELSE [radiator valve position] = 0

I may be wrong, but it would appear that Better Thermostat is not capable of this either. Additionally, I like having full control over the radiators.

In the fullness of time, I would like to have a thing on the wall that allows Granny to set the desired room temperature without having to ask for help. I expect that this will look like a traditional thermostat, but will simply output a number for Home Assistant to consume. I just wanted to ensure that I know what I am doing before I spend more dosh!