Initial thoughts after completion:
Rebirth is a messy mixed bag of a game with high highs and low lows. Despite improving on Remake in a lot of ways, I think it might actually be worse overall, due to lack of cohesion and flow + god awful pacing + convoluted nonsense. Ultimately, it still has a lot of the exact same strengths and weaknesses. I can't deny the enjoyment I'm getting from the good parts, but the review scores are severely overrating this game and overlooking its flaws in a way that feels kind of weird to me.
Like Remake, the combat, character moments, production value, and a lot of set pieces (including some absurd campy ones that are legitimately fun and make you feel like a kid again) are great. Character dynamics are very lovingly fleshed out, and are easily the glue that (almost) holds everything together.
The side-content and poorly paced bloat/filler content is actually worse than Remake, IMO. Very one step forward, two steps back. Improved the town side content only to introduce map side content which is several times worse. They improved a lot of the good sequences in the original by expanding on them (except for the Seto moment, which I think they butchered), but they also actively hurt the game by expanding largely forgettable nothing sequences from the original into unbearably long nothing sequences that bring the game to a grinding halt (every bit as hollow and tiresome as the sewer/train graveyard sequences in Remake, IMO). It's like a fantastic opening followed by hours upon hours of absolutely nothing areas/content dragging the game down before it finally gets to the next genuinely great/fun sequence/area.
The minor narrative deviations/tweaks are mostly great and welcome changes, but the dumb/childish MAJOR newly introduced narrative deviations are also worse than Remake, IMO (nothing to do with nostalgia/purism-- it's just generally sh*tty and infantile fan-fiction-level writing). On top of this, I would argue that the non-battle mechanics and gamification of everything introduced to Rebirth ultimately hurt more than they help.
Overall breakdown of areas (mostly in terms of narrative/storytelling pace):
Nibelheim Flashback/Night in Kalm: Excellent
World Map sequences: Absolute trash
Town side quests: Mostly bad
Mini-games in main story: Don't mind them (they're only bad/too much in the side content)
Chocobo Ranch, Zolom, Mythril Mines: Mediocre and time-consuming Filler that should not have been expanded
Lower/Upper Junon/Cargo Ship: Excellent
Costa Del Sol: Pretty good
Corel/Gold Saucer/Desert Prison: Excellent
Gongaga: Very contrived in how it gets there, but overall good/appreciated.
Cosmo Canyon: Looks/sounds great, but something felt off about it, it was just okay, I think.
Cave of Gi: Pretty bad
Nibelheim: Middling
Gold Saucer 2: Weak
Temple of the Ancients: Pretty good, occasionally awkward (mostly just the tail end)
All Zack/multiverse Sequences: Absolute trash. Easily the worst part of the game.
Ending: There were some elements that I liked, but just an absolute mess (especially when you don't factor in having knowledge of the original).