The baggage unloading usually starts before or at the same time as passenger unloading and it usually completes in a minute or so and the trains leave but the passenger deboarding takes several minutes and only completes some time after baggage has long been unloaded
Is not so simple.
First, it depends on your timings. You said you had deboarding set at 1.2 and boarding at 1.8, which (as the GSX setting page will tell you), translates into 2 minutes to deboard 100 passengers and 3 minutes to board.
But that time starts only from the moment the passengers starts to deboard/board which means, if you are parked on an apron, there's the time for the passenger bus to arrive to take into account.
And, if you take your time and don't open the passenger door immediately, that time will be added too.
And of course, if you wait to open cargo doors, that time will be added to the total time needed to handle baggage instead.
So, it's not so certain which one (passengers or baggage) will finish first, because it depends by many factors.
The loading time for baggage, instead, is not affected by those settings, because what is being affected by the number or passengers is, instead, the number of *carts*.
Just made a test with your settings (1.2 secs per passenger on deboarding), on the most favorable case for loading passenger, which means a parking with a jetway, opening the passenger door as soone as requested, with an half-empty airplane (6/12 people in first class, 86/162 in coach), I had two loaders with 3 carts each, and they finished loading baggage before passengers deboarded, but passengers ended deboarding only about 1 minute later.