If computers are in same network, even with different ip addresses, they still can see all broadcast and multicast traffic. This means for example dhcp.

If you fully trust your computers, and are sure that no external party can access any of them, you should be fine. But if anyone can gain access to any of your computers, it is trivial to gain access and sniff traffic in all networks.

If you need best security, multiple switches and multiple nics are unfortunately only really secure solution.

Luks FDE, and install dropbear-initramfs, configure ssh authorized_keys and rebuild initramfs. Then you can access initramfs via ssh to type luks password.

If you just rename the dir, and then find all broken symlinks in your system?

find . -xtype l

I had laptop running Ubuntu 16.04, which was running for 2273 days without reboots or anything. It was located in safe place so not even security updates were installed during that time. And it was still completely fine after all these days (little bit over 6 years). It was finally shut down when there was electricity break, and its battery failed, and I decided that it was time to retire it.

There of course were tons of updates available then, but no one forces you to install them. and in Debian system instead of Ubuntu, there will be lot less, their release policy is much stricter.

Can you access your wan ip when you are somewhere else than on your own lan?

If not, then this is probably just that your router does firewalling and nat is such order that you can access admin interface from local network via wan address.

If yes, then router has some serious misconfiguration.

Also check your bios version. I had similar problems with usb-c and fans on HP Elitebook, they were fixed with bios upgrade.

Edit: I also had troubles waking from sleep. They were caused by wwan/lte modem, I disabled it on bios and now sleep works flawlessly.

For first-time Linux users, I always recommend one of the main user friendly distributions - it is much easier to ask or look for help this way.

So, Fedora, Ubuntu or Opensuse.

Their installers all can live boot

Blog makes valid point, but why on earth there would be any current Linux distribution without usr merge?

EDIT: Especially when every major Linux distributions have already implemented usr merge long time ago.