Depending on your comfort level, you may want to do what I’m in the process of doing. I’m still waiting on parts, but this will work for my heating system.

I have old 2 wire thermostats in a few places I want to replace. I have hot water baseboard heat with multiple heating zones. I couldn’t find an existing solution that worked the way I wanted and was reasonably priced, so I decided to make my own. This only works for single stage systems and for which exhaust fans, circulation pumps, or other components are controlled by the heating system generally and not by a single specific thermostat, which if you have those old mechanical 2 wire thermostats is almost certainty the case. You could do more sophisticated, but I don’t need to.

All I need is a relay (controlled by HA) to simulate the thermostat turning on/off. I also need some way to tell it when to turn it on/off (such as a temp sensor), again lots of options with HA.

This can be done in a variety of ways, but I’m using nodemcu boards (they have wifi onboard) and esphome firmware. I’ve used this combination for a number of HA integrations so far. Near my boiler where all of the old thermostats connect will be a nodemcu board with multiple independently controlled relays (for each thermostat to control the individual heating zones).

The 2 wires that go to my old thermostats will be power supply for separate nodemcu boards, which will be in a 3d printed case along with buttons, display, and (in one room) will also include a temp/humidity sensor since I don’t already have one there. The other locations already have more sophisticated air quality sensors that include temp/humidity, so no need to duplicate, although maybe I will for redundancy.