Plex probably isn’t the best example, but yes, you can use Tailscale to create a sort of mesh network to access devices within private networks. Essentially any device that’s connected to tailscale can be contacted by other clients connected to tailscale. There are extra routing things you can do to use a tailscale device as a sort of “exit node”, but that’s the basic gist.

Seconded for simplicity. If OP is looking for complex statistics, it may not do the trick, but it’s about as straightforward and quick to set up as a monitoring solution can get.

Put this on my list of projects to dig into when I have time. I briefly tried Grocy a while back thinking I may be able to use it for inventory mgmt, but it didn’t stick. I want to say I found some other self hosted home inventory project at one point but didn’t stick with that one either. It’s one of those things that I think you have to really invest in to get ROI back, but that’s hard to do, especially when it’s more than just yourself (wife-factor) planning to utilize it that also has to buy-in and commit.