TIL about that manpage. I've always been guessing at which level in the hierarchy would contain a particular option ("Is it service-specific? Or maybe applies to all execs? Or maybe applies to all units?") and just giving up and opening them all in parallel.

Couldn't agree more. Once familiar with the big picture this page is the best starting point whenever to you want change something.

To get the big picture I read the systemd posts on https://0pointer.net/blog/archives.html But that's 10 years already! No idea what would be recommended today.

FWIW, the man.vim vim plugin does grep and some syntax highlighting. https://github.com/vim-utils/vim-man