I'd feel more safe as an admin by knowing I only have 1 port and 1 app (ie wireguard) being public than 10 with their own ports and security (ie redis and others).

This isn't about making things public, just resistant to tampering and sniffing. Yes if you want to connect networks together then wireguard is a good choice.

If your devices are already on the same network and instead you close down the firewall and you move everything into wireguard you've just moved your problem.

Exactly, far too much overhead. Also doesn't allow clients to easily connect.

It's much easier to configure once wireguard than configure on each application the ssl mecanism. Redis for example is very easy to use normally, but adding ssl makes it quite harder to setup/use.