You can configure HA to use an external database, so you could (presumably) config two instances to use the same DB. Not sure how much conflict that would cause for entities that are only attached to one of those instances, but it seems like both should have the same access to state data and history. Could probably even set one instance up with read-only DB access to limit data conflicts, although I imagine HA will complain about that.

Even with an external database, HA still uses its internal DB for some things, so I don’t think you’d ever get identically mirrored instances.

If you’re already running Pihole, I’d look at other things to do with the Pi.

https://www.adafruit.com/ has a bunch of sensors you can plug into the Pi, python libraries to make them work, and pretty good documentation/examples to get started. If you know a little python, it’s pretty easy to set up a simple web server just to poll those sensors and report their current values. Only slightly more complicated to set up cron jobs to log data to a database and a web page to make graphs.

It’s pretty straightforward to put https://www.home-assistant.io/ in a docker on a Pi. If you have your own local sensors, it will keep track of them, but it can also track data from external sources, like weather & air quality. There a bunch of inexpensive smart plugs ($20-ish) that will let you turn stuff on/off on a schedule or in response to sensor data.

IMO, Pi isn’t great for transport-intensive services like radarr or jellyfin, but, with a Usb HD/SSD might be an option.