I don’t know if, for example, a computer connected to a network running a dated version of Windows is a risk simply because it is connected to the network. Even if it isn’t being used for things such as web browsing.

Well, probably not going to be how an attacker gets in. But let’s say they crack your ssh password on the raspberry pi. Now the attacker can reach the other devices on your network, even if ports aren’t forwarded to those devices. Let’s say you’re running an smb file share on that old Windows computer. They could potentially use a vulnerability to get access to that computer.

So yes, you want good security at the perimeter (make sure your router firmware is up to date, any devices with ports forwarded are properly secured, reduce attack surface by not port forwarding anything unnecessarily, etc.). But it’s also a good idea to practice security in depth, protect even the computers that are on the LAN.

Btw, I would only forward ports to things that you want people who aren’t you to access. Something like ssh generally shouldn’t have ports forwarded for as you’re probably the only one that should be accessing it. Instead, use a VPN (I like wireguard). And when you want to access ssh outside the network, just VPN to your home first and then ssh like you’re on the LAN. You can do the same with any other service. But if you want say share a Plex server with friends and family, you don’t want to make them use a VPN, you pretty much have to port forward.