Great, so best case it takes me 1.13 days to download. Why is this so hard? The whole purpose of a package manager is to know the dependency chain of any given package, yet that information is completely useless because nobody ever considered this simple use case.

This simple use case was actually the norm back when apt was created and most people installed from CD. In fact, if downloading the whole repo is too much you can still order a DVD set: https://www.osdisc.com/products/linux/debian?affiliate=about... .

It is completely ridiculous that I need the entire repository to install a single package and its dependencies. End of story.

You can connect to the public repository or copy your own version of the repository. How else could it possibly work ?

Issue a command to apt that says "download this package and all its dependencies regardless of installed state on this machine".