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
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.
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.
Yes that's the recommended path.
Information on how to upgrade can be found in the official documentation:
https://www.debian.org/releases/stretch/amd64/release-notes/ch-upgrading.html
https://www.debian.org/releases/buster/amd64/release-notes/ch-upgrading.html
https://www.debian.org/releases/bullseye/amd64/release-notes/ch-upgrading.html
https://www.debian.org/releases/bookworm/amd64/release-notes/ch-upgrading.html
As others pointed out it might be a PITA. Depending on your installed software and configuration a new install might be easier.
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.