I think it's interesting to note that Google's "default" model now (what's recommend to incoming engineers) is the fully trunk-based monorepo, in a Perforce clone called Piper... But with a Mercurial based interface for actual development.

In your individual checkout you can have arbitrary branching structures, then it just gets squashed down to a linear model when you submit to the monorepo.

It's a bit annoying from a technical POV that there are two separate VCS, but the conceptual model works well IMO.