I like the idea a lot and the use case you describe perfectly matches my personal use case. The HestiaPi is just smart enough to run autonomously but my main automation is running on a server somewhere else on my network.
Some additional things I like about using a microcontroller:
- it’s not as big of a deal to keep up with updates; as a security engineer I’m still uncomfortable having a machine on my network that shouldn’t be updated periodically
- near instant start/reboot times
- smaller size
- less power
The software will, of course, need to be completely rewritten and depending on which ESP model you choose you might run up against storage limits if you have to import too many libraries (MQTT, sensor, etc). Definitely pay attention to that as you choose the model.