I’m not sure I follow. You can terminate your cables into wall mounted sockets just like your electrics. What’s the issue with this?

No, because a switch can’t route traffic from one point to another. Specifically, it can’t do NAT. Without NAT you can only have 1 device connected to the internet.

If you move it and it changes behaviour, that’s some sort of bad mechanical contact. Cabling, socket.

Are you sure you know all the rooms in your home? Is the house bigger on the outside than you can account for on the inside?

Are you behind cgnat?

Can you post a screenshot of everything? Can’t tell what the link is. Your PC to the router? Your router to the WAN?

Use vlans to create a dmz you can place a server only for public Internet facing stuff.

If you’re worried about privacy and security you can always DIY. If you are able, it’s always the best option for a NAS.

Try connecting your computer directly with a cable 9t at least test your Internet speed right next to the router. Whatever the result is the best you’re going to get from your ISP.

Yes, that’s very true. But worth remembering that while you can usually get away with cat 5 for 1gbps, that’s only up to a certain length, depending on the quality of the cable. If your cable is in or around the threshold for that particular cable where noise picks up, you could get inconsistent speeds like OP is.

Inappropriate cable?

You contradict yourself on paragraphs 5 and 7. First you say it’s reachable from the outside then it isn’t.

DIY always better for an all around server which is a nas but also a application server.

I have an old i7-7700T with 32gb ram in a fractal node 304 case, running Ubuntu and 3x4tb drives in zfs raidz1. There’s little it can’t handle.