I'm kind of conflicted on the final arc. It feels like an anime movie, spectacular as hell and well done in general, but ultimately filler. I mean, and big spoiler warning here, it doesn't matter. It all takes place in an alternate universe and everything resets when it is done. All the new content sprinkled in what the base game covered was nice, and really enhances the experience.
As for the Royal exclusive story, I really loved it. I LOVED the Garden of Eden imagery in the final palace. It was super thematic to the story of the Phantom Thieves. I mean, Joker's ultimate Persona is literally the Devil, and in the end he frees humanity from living as subjects of a paradise and in exchange grants them freedom to live and grow on their own. It's pretty much 1:1 with the story in Genesis and Paradise Lost. Maruki's final Persona was even Adam. (side note, I know his persona wasn't first man Adam, but Adam Kadmon, a sort of Hebrew interpretation of the collective unconsciousness, a very fitting persona for him, but the name fits with the Eden imagery as well, a great double-dip of symbolism.) Kind of makes me wish either Joker or Sumire gained the persona Eve to match it, rather than just the action scene at the end, but that might have been too on-the-nose or just a copy-paste of Satanael blasting Yaldabaoth from the original ending.
I liked the expanding of Akechi's confidant. Keeping up with it kind of changed him from being just another villain into a dark counterpart to Joker. If the Light Yagami parallels with his characters weren't already enough, now it feels like in another universe, they might have been friends. It felt nice to play alongside the REAL Akechi in the final palace, with all his bloodthirsty serial-killer glory on full display. I just wish they carried it over to the other confidants. They expanded the twins a bit, (still no romance option, sadly) and they have some comedic scenes, but poor Sae is just left on the sidelines to get dragged along automatically through the story as before.
Mementos is just...better. Not much to say on it as it was improved in EVERY WAY. If you hated Mementos before, its likely you tolerated it a lot more this time, and if you liked it, you LOVED it now.
Sumire as a character is a nice addition. She fits in well with the rest of the Phantom Thieves, and the plot twist of her character is pretty original and unexpected. Maruki, while basically just a less evil Aizen from Bleach (he even has very similar powers, not just his looks) is a better final villain than Yaldabaoth. Yaldabaoth was a faceless God of Control, who wanted to dominate the world just because. Maruki had a personal reason to do what he did, and rather than control people in a prison he would rather do so in a paradise, making him infinitely more sympathetic.