Yes, there are billions of different setups, but there are a few basic ones, and you could start by solving one or two, before solving the generic case. For example, one could provide just a cluster setup for data replication. No fancy distributed data models, just copy the data around in the cluster, if a new node joins copy that data to it, if something fails, handle the failure. Postgresql claims to have multi-master setups, so this is really about the node handling and copying the data around.

After that, you could introduce locally distributed cluster. Later on you could introduce geographically distributed cluster setups. But just because the later is very complex, does not mean that you can't start with the basic setup.