It's great to hear from you again! (And woah, that's a staggering amount of battles). :O
It seems that you still have one more big challenge to complete, and it's related to the plushies! There are 9 of them in total, hidden around Artistry Highway - one for each of the 8 party members, plus Arianna. If you tell me which ones you're missing, then I can give hints or tell you where to find them.
You can access the Sky Abyss from the summit of Mt. Dramatica, which is to the east of Wikipedia. :3
The DLC doesn't really change anything about the base game, before or after beating it; except for a couple of places (like one of the street stalls in YouTube) now being able to sell furniture for your house in Etsy.
By the way, did you ever join the BoxxyQuest Discord server? It's not very active at all, but I'd still love to have you there. :D