Alternatively, if you're not stuck on Linux, just install OpenBSD or NetBSD. Both will try to install X by default, but you can easily disable this in the TUI installers. After a reboot you'll go straight to a shell with very few daemons running (and certainly no systemd). Install the packages you need and start the daemons you need from there.
Imagine having the entire output of `top` fit in a terminal window with empty space at the bottom.
Everything that's used should be cached in RAM, anyway. Everything that isn't used, well, I prefer that not to be in RAM. These things don't exactly have gobs of it.
And why would I want to remove the SD card after boot? Now I can't reboot the system, and it won't reboot if it crashes?
> why would I want to remove the SD card after boot
It's unnecessary to remove it. It's more about reliability. SD cards are notoriously unreliable, and it could potentially fail after boot. Coupled with the previous point, it has happened multiple times that I wanted to login to troubleshoot the system, but cannot because the basic system binaries like `ls` are not cached in RAM (as you said these are not used for the normal operation of these unattended systems).
This is when RAM disk is very helpful: as long as the system is powered on, I have reasonable confidence that I can login and see what's going wrong. It's mostly because RAM is way more reliable, faster, and cheap enough to "waste" a few dozens megabytes for the entire base OS.
Also there're diskless PXE systems where you just load the OS from network only once upon boot.
Imagine having the entire output of `top` fit in a terminal window with empty space at the bottom.