Nice, good luck! Did you happen to try setting up GNOME so that it behaves more like a tiling WM? There are some extensions for that, like Tiling Shell, SpaceBar (also here) etc.
But that would be probably not as fun as exploring that low-level stuff.
Typically you shouldn’t use repositories from other distros in debian, only use repositories meant for that purpose. Packages have specific requirements, and their versions will probably differ between distros.
The same is true of Cinnamon, Mate, Xfce, KDE, and Gnome. If i install Debian and it doesn’t come with one of these listed in the aforementioned file (and it doesn’t), i have no idea how to get packages from that repository unless i can also find a downloadable .deb file and it has no dependencies from unknown repositories, or i download the entire desktop environment i want just a few packages from.
apt searches for packages listed in repositories defined in /etc/apt/sources.list, like you said, but nowadays mostly in .sources file inside /etc/apt/sources.list.d. In a regular install you should find there a debian.sources file with the main debian packages, which include all DEs you mentioned. For example, try: apt update && apt search cinnamon. You can browse the packages here: http://deb.debian.org/debian/pool/.
Repositories usually include sets of packages, not a single package/software. So don’t expect to find a repository for XFCE, another for GNOME etc. All these packages are available in the main Debian repo.
You can add other .sources files to activate other repos. That’s actually what extrepo did for you, you should find a new .sources file related to librewolf.
Regarding xed, that’s indeed not packaged in the Debian repos. You could try manually downloading the Linux Mint xed- packages (link), installing them with dpkg and see what happens, since that wouldn’t replace core debian packages. Or simply build them manually, there are even instructions for building a .deb package: https://github.com/linuxmint/xed#installation.
For context: i plan to install Debian without a DE and just get what packages i want from across several DEs. This will be hard to do if there are no software sources for apt.
You can try installing specific tools of some DE, but possibly that will download lots of libraries of that DE required by that specific tool (which shouldn’t be a big deal anyway). You can list the dependencies of a package with apt-cache depends ... to see what you need. There are also lots of software not designed for a specific DE and thus have less dependencies, maybe check what folks using minimalist window managers (like dwm, i3wm etc.) suggest.
I love those buildings and live in one, couldn’t be happier. Super nice view of the city, and very affordable since where I live many of those buildings still belong to the state.
One advantage many overlook is the amount of natural light, what a.o.t. has important benefits to the mental and physical health of the population. According to Wilfried Stallknecht (an architekt behind many projects in the GDR), the buldings should keep a distance that guaranteed that every appartment had, on the 22. February of the year, at least a room with 2 hours of sunlight. If you walk in those planned areas you see the huge space surrounding the buildings, with lots of green areas, parks, playgrounds and so on.
Definitely vibecoded, sad to see that stuff replicated here.