From a non(QF)-partisan point of view.
Welcome to LIFE,
wingnutt - no-one
ever said that it would be "fair", although most people try to live by a mutually acceptable set of guidelines that ensures harmonious living with their peers.
The seniority system that you are fortunate enough to have in place, but are complaining about, is the very SAME one that protects YOUR position, when someone with more experience than you joins.
It's the SAME (seniority) system that will, again, protect YOUR position, and keep you moving up the ladder, should you have a medical condition or accident that prevents you from doing any flying for a considerable period.
You say that you're still trying to work out
how flying a metro or bras for airnorth or skippers can count as two years towards your seniority for an FO slot etc, when most GA people have done exactly this already.
...whilst people who have been flying Metros or Bras with 4 and 5 times your experience, are still trying to work out why THEY have to slot in BEHIND
you.
Wiser heads than your's have sat down and arrived at the structuring of the seniority you are blessed to have.
Protect it, because it sure as Hell will look after
YOU!