Now if a consequence of that could be that one (as an author of a piece of not-yet-debianized software) can have the possibility to decently build Debian packages out of their own repository and, once the package is qualified to be included in Debian, trivially get the publish process working, that would be a godsend.

At the moment, it is nothing but pain if one is not already accustomed and used to building Debian packages to even get a local build of a package working.

Oh, yes. This seems like nothing short of necessary for the long term viability of the project. I really hope this effort succeeds, thank you to everyone pushing this!

There is some nuance to this. Adding comments to the stated goal "Everyone who interacts with Debian source code (1) should be able to do so (2) entirely in git:

(1) should be able does not imply must, people are free to continue to use whatever tools they see fit

(2) Most of Debian work is of course already git-based, via Salsa [1], Debian's self-hosted GitLab instance. This is more about what is stored in git, how it relates to a source package (= what .debs are built from). For example, currently most Debian git repositories base their work in "pristine-tar" branches built from upstream tarball releases, rather than using upstream branches directly.

[1]: https://salsa.debian.org