@Roke Beedell: I am very glad to see thanks to these discussions that a "Zarro Boogs Corporation" was founded and that Bugzilla is now technically independent from Mozilla. That being said, I do think it is quite optimistic to plan an upgrade to a version which is pre-beta, in particular when Bugzilla has no changelog. Even version 5.9.1 took 12 months more to release than the planned 1 month.🙄
It should be noted that "Bugzilla Harmony" is not a product, but a codename for a new major release of Bugzilla. The codename signals that release’s promises, but also the efforts this ambitious project needs.
I sure hope to see Bugzilla 6 one day, but the last truly major Bugzilla release was almost a decade ago, a decade which was even harder for Bugzilla’s biggest user than for Perl. So while I applaud your intention, I am afraid this is indeed pretty premature.
Thankfully, there is no need to demonstrate a feasible alternative to GitLab; as Bugzilla is already in use, the onus is on GitLab proponents to demonstrate superiority of its ITS engine, and that request―which is mostly about code changes―is far from doing that. And even if a migration was needed, GitLab would not be the first option to consider (Atlassian Jira, Redmine, YouTrack…).