Ok so coming back here now, I think I got all the "main" endings for the game, with a few of the extra/joke ones. Took me 33 hours.
To be honest the endings themselves didn't have as much impact on me as I expected, but I really, really enjoyed the journey up to these endings, and I'm the type of person who cares more about the journey than the destination anyway. A lot of fun twists and turns, with some interesting attempts to add more layers to stuff I normally wouldn't have taken that seriously. Route B was a little boring at first but I thought the pay off around the end was good and things only got wilder from there.
Talking about gameplay only, I wasn't too fond of the side quests. I'd put them in the same tier as Final Fantasy XV, which I think also has really bland and tedious side quests, but the writing side of them in Nier is much better, with the NPCs having more personality. A lot of the side quests ending in some kind of existential crisis or dilemma got a little odd though, not that I didn't appreciate the attempt at examining the overall themes the game had from a bunch of different angles. I think RPGs can be very inconsistent about keeping the writing in line with their themes and examining it in different ways (though I think several RPG developers have improved considerably at this in the past few years), so ones that get it (mostly) right on keeping the writing focused earn a lot of points for me.
The open world I'm a little torn on. I thought the scale served the scenario well, and traversal wasn't too bad, but I didn't really develop any kind of "this is in the game and I (don't) enjoy it" feeling about it. It was kind of a means to an end for me and never really stood out in my mind the entire time. I may just be desensitized to them at this point and don't really care either way unless the developer executes on it in a way that really stands out from my usual expectations.
The past 3-4 years have been really good for RPGs -- both Japanese and Western-made -- and I've actually really liked most of the ones I've played (the most "meh" of the ones I've played in that time period being Tales of Zestiria, Final Fantasy Type-0 and Final Fantasy XV, but Tales recovered from that well and I guess I'll have to wait three years and see how Final Fantasy VII turns out, lol). For better or worse, this is probably the most interesting one among the RPGs I've played in that 3-4 year period, and I don't regret picking this up.