Retrieve thermostat setpoint in a scene trigger

Good evening,
I already use Gladys for my heating and finally looked into the « new » Thermostat developed by @pierre-gilles a few months ago.
To make it useful, I need to be able to choose a value with this thermostat, add one degree for the high setpoint and subtract one degree for the low setpoint (e.g. if I choose 20°C, the heating turns on at 19°C and turns off at 21°C).
So I make a scene in which I retrieve the thermostat value, send the operation via mathjs and then…
Mathjs doesn’t accept my ‹ Thermostat › variable even though it works with numbers and then…
I don’t know how to choose the result in « control a device » towards a dummy MQTT device.
Does what I’m trying to do match the problem mentioned above? Is it not supported? How off-base am I?
Edit: I saw here that @bertrandda had worked on the topic — is it better that I shelve my project and wait?