Welcome then.
Well, as OH is a pretty flexible set of software, there’s no definitive setup guide to THE architecture.
But the majority of people who are deeper into OH based home automation will be running a setup similar to what you outlined, so I’d give those ideas a general thumbs up.
Run a single OH instance plus a MQTT server such as mosquitto on a single, central RPI. Deploy slaves to talk MQTT to the MQTT server on the central one, but DON’T deploy slave OH instances. Slaves can be anything to talk MQTT, e.g. Arduinos to ‘natively’ talk or RPIs to run simple scripts like this or this one, or whatever device you already have working in your setup, or any mixture thereof.
Inside the central OH instance, you would need to configure items for every “thing” (sensors and actuators) you run, including those physically attached to your slave servers. For those “remote” items, you would need to define them to be of MQTT type. You need to run the MQTT 2 binding on your OH RPi for this to work, see link for examples or search this forum for more.
