Very much in line with your opinion, to me its a 10/10 game most of the time with some mild hiccups that take it down a notch for a couple of moments.
I very much enjoyed the expanded story and much like how you put it, I was very impressed about how they fleshed out the characters turned out to be. Much like in FFXV, you feel for them. The fact that the game its story/character driven makes the linearity of the world design make sense and fit well. Not sure if its the direction I would've picked if given a choice, but it works well here since its done by design and its well written. Maybe the ending is the more divisive point of the game, but I personally like it quite a lot, its very meta and very telling of their intent to deviate from the original story in further installments. I hope that is the case, I like it when remakes are also a reimaginings, because certain elements dont translate well after 23 years. Dialogue being one of them, it threw me off a bit in the begining of the game how Barret is presented. Very much in line with how the original character was portrayed, but im not sure if that was the right call for this interpretation of the game.
As for the more technical aspects, well the game is gorgeous in spite of a few instances of frame drop, but the gameplay- specifically the battle system- to me is the best in the franchise, and I do mean that. I mentioned this since the demo came out, I loved how the mix the active battle system of modern RPGs with the classic ATB system of old, I hope this is a model they reincorporate for future FF games. I absolutely loved it and find it very dynamic.
I think the filler content is fine if they just make it side quests and design the levels better. Overall I think this speaks on the poor design of the side quests. Normally what you would have is a few unique areas in the game which you hear about, maybe briefley visit but do not properly explore. Exploring them properly requires a few key items and levelling up character to deal with the stronger fiends. This is also where you would find unique items and materia and a chance to pit them against stronger foes.
The abandoned Shinra research facility/
all could have been designed to host what you're talking about and include additional story tibits and world building.
They could have introduced something like corrupted materia, an attempt to recreate summons gone wrong. Essentially the dark aeons from X.
I would have gone further with the world interation and let you cast magic outside of battle on specific targets (like how you could hit/shoot specific targets). It would be a natural way of unveiling hidden areas. Revisiting a location once you have mastered time materia so you can cast stop on a propeller that is preventing entry to a deeper level of the research facility, or using aeroga to displace junk, blizzaga to freeze a body of water preventing passage, Thundaga to power a now defunct piece of machinery