Ooh… Apostate. First things first Pet Apostate can and will work well enough to do everything ingame except Callagadra/Crate of Entertainment (Mogdrogen not tested)
Here are 2 possible endgame paths:
BlightFiend based: Fat & Fwuffy - Pet Apostate
Reap Spirit based: Friede - Pet Apostate
Both can farm Gladiator Crucible easily and do SR 75+ on Ultimate.
Skeletons are however a no go for endgame stuff. That said, since those builds above require specific items and stuff to work, this is what we will do in the meantime:
Lvl 80: https://www.grimtools.com/calc/gZwJlEnV
Lvl 90: https://www.grimtools.com/calc/D2p89yJ2
Lvl 100: https://www.grimtools.com/calc/w26RmLA2
Disregard the items for the time being. Work towards this as a budget setup: https://www.grimtools.com/calc/q2mlmjWN
Once you get to that, then work towards getting the items in one of the two endgame Apostate builds I posted. You can farm Skeleton Key dungeons for the random legendaries.
If you have questions, feel free to ask 