@ottok Nice to see some discussion about this, as a fellow DD. I would love to see the Debian way integrated into salsa.debian.org a lot more, e.g. use GitLab as the back end while maintaining a lot of the same interfaces, e.g. email interactions with bugs.debian.org. There has already been good work on improving the salsa/gbp workflow. DEP-18 sounds good too.
In @fdroidorg we actually fetch upstream source code using Git, then packaging in maintained in our GitLab.