@nyanide@lab.nyanide.com I see, on Slackware this basically would be:

# build packages, mount USB drive

cp /my/repo/of/updates/*.t?z /mnt/usb/

# mount USB to new machine

upgradepkg --terse --install-new /mnt/usb/*.t?z
This applies equally to official Slackware packages, and any custom packages, from SBo or otherwise.

Of course on Debian you can do the same, with apt install /mnt/usb/*.deb. I think if you do it this way, the dependency resolver of apt won't go crazy, because it can order the .deb installation to keep dependencies satisfied. Do not try to install the packages one by one though, because you will have to do it in the correct order then.

Unrelated recommendation, if you're thinking about Debian, check out Devuan, for a more sane community, and a more sane init system. Devuan is still down-stream of Debian, so Debian shenanigans might impact them, but at least you have a layer of sanity-checks between you and the Debian project.