That's what the AppDaemon project is about - https://home-assistant.io/docs/ecosystem/appdaemon/ - it let's you do the automation rules inside code instead of inside yaml. You'll still need/want yaml for basic device connections and grouping, but doing complex automations can be done easier in AppDaemon.

They recently introduced "straight python" components i believe, so you can just write in that.

Additionally there are multiple apis you can use to interface other than the front end at https://home-assistant.io/developers/api/

Also the HA front end just calls those backend apis, so if you "emulated" those you could use it as a drop in for some other backend (in theory)

Thanks, I'll check that out!