This is one of those situations where Linux drives me nuts.
So I want to install a minimal version of Debian. Once I install it, I need to install network-manager. But I don't have home internet - I have an iPhone hotspot. And I can't connect to the iPhone WITHOUT network-manager. Ugggh. I do have a Raspberry Pi 3B+ that could work as a WiFi-to-ethernet passthrough, but finding information on setting THAT up wasn't worth it.
So after a lot of digging, I've found a way to do it. Unfortunately, it requires another computer already running Debian. If you aren't already a paradoxical member of the Linux clan, find something to install a non-minimal Debian on, or use VirtualBox.
The first answer of this link contains most of what is necessary. Make a directory first, cd into it, and then run these commands:
apt-get download $(apt-cache depends --recurse --no-recommends --no-suggests --no-conflicts --no-breaks --no-replaces --no-enhances network-manager | grep "^\w" | sort -u)
dpkg-scanpackages . | gzip -9c > Packages.gz
Then copy it to a USB drive, and copy the directory from the USB drive over to the computer in question. But wait, we aren't done yet. You need to make apt recognize the little repo you've made. And of course, it isn't easy to find out how to both add a repo AND skip the security check. Add this to /etc/apt/sources.list, and comment out the other sources:
deb [trusted=yes] file:/the/repo/directory ./
...and then the usual "sudo apt update". Congratulations, NOW you can install network-manager!