Honestly, just use Debian. It can run under 200MB of RAM (default install), so it beats all distros on the list except for TinyCore and SliTaz, and it actually has packages.

I believe OP has made some mistakes when pasting the text

[Diagram]
Oracle VM: Linux
External IP: 192.0.2.1
Internal IP: 172.16.0.2
 |
 | GRE Tunnel
 v
Home Server: Linux
Internal IP: 172.16.0.2
 |
 |
 v
Firewall: FreeBSD
Internal IP: 172.16.0.1
External IP: 192.0.2.2

[Text]
Home Linux Server GRE config:
$ sudo ip tunnel add gre0 mode gre remote 172.16.0.2 local 192.0.2.1 ttl 255
$ sudo ip link set gre0 up
$ sudo ip addr add 10.100.10.1/24 dev gre0


Home Linux Server GRE config:
$ sudo ip tunnel add gre0 mode gre remote 192.0.2.2 local 192.0.2.1 ttl 255
$ sudo ip link set gre0 up
$ sudo ip addr add 10.100.10.1/24 dev gre0

Firewall:
nat on igb1 inet from ! (igb1) to any -> (igb1:0)
nat on igb0 inet proto gre from 172.16.0.2 to any -> (igb1:0)
rdr pass on igb1 inet proto gre from 141.148.84.178 to (igb1) -> 172.16.0.2

Apart from being open source what is Linux?

The codebase in git.kernel.org’s torvalds/linux.git

Could I not create my own operating system that is different to windows or Macos and call it Steve

Of course, in fact many people have done so: TempleOS, MINIX, SerenityOS, etc.

What do you mean by «support»? In my Debian install I created an encrypted partition + LVM and I can hibernate without issue. I believe Ubuntu has an install option for encryption, so I think it should also work.

Flatpaks for graphical apps and guix for CLI programs and libraries.

The porterbox is a HiFie Unmatched.