I’ve never used HA OS but from what I know you can’t get full access to os command line without major tinkering. In such case you need a device (e.g. another RPi or an ESPhome compatible module e.g. NodeMCU). For a linux install (e.g. another RPi) you can find pretty complete and detailed guide here:Zehnder ComfoAir CA350 integration via serial connection (RS232) and MQTT
and for ESPHome installation check out the github of that project https://github.com/wichers/esphome-comfoair
Depending on which option you choose either come back to me and other contributors on the “share your project” topic (first link) or get in touch with the author of ESPHome port (second link).