A few months later I have come to the conclusion that "trainings" are not a good concept. It's basically showing a program which can also be a video. And that would be T214522: Create tutorial videos about phabricator.wikimedia.org already.
Instead I'd see regular, general Phab Q&A video calls (one in very late UTC, one around noon UTC), or "Office Hours" as some people call this, more useful.
Similar to existing "Documentation Office Hours" (which I recently joined and it was quite interesting and insightful to attend.)

This is in addition to https://www.mediawiki.org/wiki/Talk:Phabricator/Help (some people simply prefer to talk in person instead of posting things in the very public, especially when you are unsure about some stuff). Any invitations to office hours should also include a link to that very help page.

Two recurring WMF-internal Q&A sessions per month set up on a Tuesday 10:00 UTC and 21:00 UTC. (Might expand at some point.)
Going to send reminder emails to internal wmf-optional@ before each session including a link (as some contractors don't have access to the WMF staff calendar).
First edition took place with two attendees.
Going to resolve this task after the second edition has taken place.