The trouble with a seniority system is that the false premise is that it is fair and equitable.
Many grey hairs have been made to get it in place, it does give all a shot at promotions/bases/days off even to the dullest knife in the draw.
Possibly evens out the "favouritism" process, but in the event of take-overs and amalgamations between Companies, causes bitterness and hatred. (Ansett amalgamation in my case).
How you account hours from GA to long haul with a sleep component is above my pay grade.
An overall look at all of anyone's experience will always be fraught with opinions/bias and other imponderables.
There is no short cut to be had for some folks, and some folks get a lucky break, I got a call from the flight manager, no face to face interview, knew his sons, they were in great need, had a job in all sorts of places ever since.
