Where are you running du -sh *? (I.e. what directory, are you definitely scanning the whole file system?) I'm sure it's obvious, but can never hurt to check!

What does du -sh / show? (Generally, the * glob pattern in the shell will not match hidden dot-files, so is it possible they are being excluded?)

If you're using the AIO image, backup/restore can handled for you, so no need to worry about the manual steps involved. Or if you're using a VM, a backup can take the form of full system snapshots, so also no need to understand how data are stored. Granted it's always helpful to know what your running, but not necessarily requisite, even for backups.

Absolutely. I actually have an upgrade already planned, but it's just that it's not because I can't run VMs, it's more that I want to run more hungry services than will fit on those resources, whatever virtualisation layers were being used. The fact that it's an easy fix to more a VM/lxc to a new host is absolutely it, though.

Am I looking at the wrong device? Beelink EQ15 looks like it has an N150 and looks like 16GB of ram? That's plenty for quite few VMs. I run an N100 minipc with only 8GB of RAM and about half a dozen VMs and a similar number of LXC containers. As long as you're careful about only provisioning what each VM actually needs, it can be plenty.

Grew up on Armada and Away Team, but of those, Away Team was definitely my favourite!

More recently played Elite Force, which was also pretty dang great.

Onlyoffice does now have an ARM version, I run it on a raspberry pi 4 (integrated with seafile), and it works fairly well. Can't vouch for how much power it needs, to say if a pi 3b+ will be enough, though. Pretty sure it's lighter than collabora, since more is done on the client side.

I agree with the other commenter that suggested cryptpad, though. If all you're after is a Google docs like collaborative experience, cryptpad is brilliant, and much more resource friendly. (The office editor it uses is also a slightly modified Onlyoffice, so almost exactly the same feature set)

That shouldn't be happening. I use mine as a desktop all the time, Firefox is pretty snappy, with a whole bunch of tabs open and extensions. Sorry I can't give much more insight into what's happening, but it's definitely not right.