Finally found a good solution for local access to my Remeha hybrid heatpump: https://github.com/mcc-tweakers/esphome-remeha-confida
It works over BLE, so your Remeha device needs to support that, or you need to buy the Remeha Home Hub device (which I did). All devices supported by the latter should be supported by the aforementioned ESPHome component (I have an Elga Ace All-in-One).
Still need to work on making it work as a proper thermostat within Home Assistant.
Also, make sure to clone the repo locally, as I've personall been hit by Remeha cease&desist threats before…