So basically decided to get back into the game after 6 years, been busy with work and league of legends...(rank games are boring to me now)
Decided to play my marauder facebreaker and working my way to ancestral warchief chieftain, lots of things are confusing me...narrowed it down to couple of questions.
Feel free to answer whichever u can! appreciate it.

1. So there are no more difficulties? no more normal, cruel, merciless? just advanced difficulties as you progress in acts? also I remember 6 years ago killing few bosses (not sure which ones..) but seems like I'm already in the Epilogue? Final act? (automatically unlocked?)
2. Ascendancy class? decided to Marauder->Chieftain. Now I did all the trials of Ascendancy up to the point of 1/6 Trials for the Eternal Labyrinth. Is there any fast way to do this? or you just keep doing maps until it appears randomly?
3. Are maps best way to late game farm now? are any bosses worth for farming drops?

1) correct.
2) trials are harder to get on standard, on leagues we use global channels to share them
3) There are loads of options for end game farming, mapping, delving, incursions and some bosses like uber elder all can generate profits.

2. Global #820

Extra:
- Betrayal has a lot of nice crafting recipes, so I'd suggest starting new in this league if you're into that.

- If you don't have time or dislike restarting, standard is pretty good this league, too.
There is Incursion, Beastiary, and Delve now, so you actually get to experience three past leagues that you missed

i'm gonna pm you, it seems you've been gone a very long time and need to basically scrap 90% of everything you knew and relearn