Indeed wombat, and thanks -
In our case if there had been 2 sets of steps (as there should be) then the first bus could have gone out as you say, with most of the pax to get them on-board and seated, even with fuel still going on, and then popped back to the gate for the stragglers...and hey Presto! - doors closed OT, or early - Skippers happy.
Simple really - not rocket science to achieve OT departures - Oh wait, yes! I helped write the manuals about 40 years ago LOL.