It looks like the apt-mirror script project is dead. The repo directory structure has changed since 2018 and when I added 20.04 to the mirror it did not download a bunch of stuff.
So, I'm testing debmirror. Which also has problems, but is still being developed. The version of Debian I'm using on guava uses an old version of debmirror, so I grabbed the script off of the git repo and installed it locally.
Along with a wrapper script it seems to be much easier to use than apt-mirror was. Fingers crossed it works well when I start to run it as a cron job.
With the new mirror I've had an initial success with a 20.04 PXE build of one of our new lab NUCs.