Stuff has changed a lot, Make sure to get Ashes of Malmoth & Forgotten gods, otherwise game is not worth playing imo
Yes it is worth exploring. Tons of hidden bosses and quests. Grim Dawn's scale compared to other open world games (Possible spoiler)
Yes. You always craft relics. And other gear can be crafted too. Leveling gear, stepping stone gear and end-game gear. Each blacksmith provide certain bonuses to crafted gear.
Yes, there are. 5 skeleton key dungeons with a separate boss. Ravager, Callagadra, Lokarr and Crate and Mogdrogen are the “world” bosses. Also there is shattered realm. A sort of version of greater rifts from diablo 3.
Does it matter if it is vertically you will still not “see” it like that but yes there are many dungeons.