Laeon val Observis
First of all, welcome back.
This link is the best for your inquiry on BiS (Best in Slot) gear. Be warned that plenty of the stuff in that list is endgame grade (locked behind the toughest battles, which are mostly in the Inn/Arena at the Edge of Time. It's the crack in the sky of Falconreach's main room), so be prepared to work your luck and tactics to the limit of what's winnable. Since you've been out for a long time and most likely not up to date with how stats affect defenses, resistances, damages, etc., this link should get you up to speed on that. DA holders get to stat (un)train at no expense. Our pet dragon also received a major overhaul fully designed to capitalize the Manual Pet actions toggle in the Options menu, making them invaluable and powerful for a variety of situations.
DF has undergone plenty of storyline updates, so it might be a bit jarring. However, Books 1 and 3 have a feature called timeline which serves as a guide as to where you are in a saga you're completing. Rest assured, it's only a guide and will not lock you into a single story. Book 1's timeline is mostly conceptual for now, serving mostly as a guideline for the developers once Book 3 is cleared and Book 1 can be streamlined with their full attention. You may find early Book 3 storyline quests changed irrevocably from how you last remembered them. It's part of the dev's streamlining of the storytelling. The plan is Book 3 revisions totally replace their previous incarnations, while the upcoming Book 1 renovations will have the originals available somewhere.
For seasonals, it's recommended to purchase the Storybook Collection house item once Black Friday comes (pricing will be halved from 5k DCs to 2.5k DCs) so you have unlimited time to catch up to seasonal content without draining too much on DCs, but no one will stop you from purchasing it now at its full price.