A very constructive way that folks could help with this migration would be collect various "typical" pull request workflows and document their implementation as well as pros and cons. Gerrit is highly opinionated in how to use git for code review. GitLab/GitHub provides a lot more flexibility, which can be nice, but can also be a burden. If we end up with 200 repos and 50 different workflows that will in itself be a large impediment to some of the hoped for gains from moving to a more widely used git hosting and code review platform.
I think in an ideal world we would end up with consensus on a "preferred" PR based workflow that projects would be encouraged to adopt. Forcing a single workflow across all projects is itself not ideal, but having a standard process will make it much easier to explain how project/repo X differs from that Wikimedia standard practice and why.