Following the comments and response so far, I looked around quite extensively over a broad range of linux distributions. Arch, Alpine, Debian, even Gentoo booting them up and seeing what worked and what didn’t.
I found AntiX which appears to meet many of my requirements.
AntiX as a base install comes in at under 2GB HDD and 300MB RAM. By using lighter desktop environments I can push this down even further, admittedly sacrificing some usability.
It does not, sadly, have access to the AUR as it is Debian, however, there is the Sid repository, which I guess will have to do. It comes pre-loaded with RSync, LibreOffice and Firefox (which I will be booting shortly).
Even with the base of 300MB, I’m not sure I could manage to run Whonix through it, so I’m going to have to look at a different method to achieve my goals. If you have more RAM, this would be idea.
One sledgehammer coming up! Of course you’re exactly right, I had forgotten about LFS somewhat. It’s not for the fainthearted, it’s a one stop shop of how to linux like a boss.