Hosting for yourself so you can access your content outside your home is usually the use-case, use WireGuard for that though (checkout headscale) along with virtualization, VLANs, etc.
Hosting for a group of friends and/or family can usually be ok, assuming that is a well known and restrict group.

Hosting for the general public from home is usually not recommended, use a VPS for that. Bear in mind you’ll likely be liable for what you host, one way or the other, depending on your jurisdiction.
If you store content (files others may upload like movies and photos) you may be responsible for that (i.e. is that content legal in your jurisdiction?).
There may be a legal distinction between the server’s geographic location and the entity responsible for it - but in your case it’s the same, so, again, beware.
Just linking to content deemed illegal may get you into trouble.
Putting the site behind a login-only page and vetting account creation could mitigate (or exponentiate) this.
Anyway IANAL.

What do you want to host and for whom?

Maybe headscale will do better?

virt-manager supports, at least, kvm and lxc/incus, so you should be fine.

I’m happy with FeedBro (browser extension), very customizable.

Microwave. Strong magnets. Drills. Scrapyard shredders. Volcanoes.

Does Dokuwiki count?

You’re probably thinking of PairDrop (which also allows extranet).

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You can OnionShare if you’re worried about privacy.

Not being an expert it seems as though your setup is windows-centric, whereas KVM tends to shine on linux.

Or headscale.

Dozens… yt channels, tech blogs, changelogs, etc

I use a cryptomator mount and sync that to whichever cloud i want, but the un/mounting is manual.
If you need full disk encryption look at ZFS and snapshots.
Do you need to backup everything on the disk?

VPN: Headscale is an open source, self-hosted implementation of the Tailscale control server, which itself uses WireGuard under the hood.

For the file share, i’d say separate into two groups/VMs probably.
Whatever you might want to self-host publicly (are you sure?) keep it isolated. Its own VLAN, IP/host/subnet, container, VM heck its own hardware if possible. Or use a VPS and only self-host your private stuff in your LAN.

For what you should host: that’s up to you. I’ve heard jellyfin’s used a lot for media stuff.

OK

Atm main sys is a ZFS RAIDZ1 on 3 SSDs
Weekly-ish backup onto 1TB external HDD.
Sync encrypted important stuff to Cloud.
Syncthing some stuff to smartphone.

Which old Thinkpad handles all of that?

freenginx

I kinda like joplin’s groupable notes… then again “flat” is in the name… maybe… interesting…

Joplin synched with syncthing (or Synchthing.fork on android).