Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run. Debian provides more than a pure OS: it comes with over 59000 packages, precompiled software bundled up in a nice format for easy installation on your machine.
Use the archive?
deb http://archive.debian.org/debian/ stretch main contrib non-free deb http://archive.debian.org/debian/ stretch-proposed-updates main contrib non-free deb http://archive.debian.org/debian-security stretch/updates main contrib non-free
I gotta ask though, why?
This is the way.
Tbh... Itd probably be easier to back up files and settings and do a clean install of 12. If you try to upgrade in a chain like what you are wanting to do, not only is it going to be very tedious, something is virtually certain to break in a way that will make it much easier to just install from scratch.
If OP has good backups, they should try to upgrade from 8 directly to 12.... For science
It’s maybe possible but pain. I tried once and it lasts hours and hours. It’s easier to install new and migrate your configs.