For you, a little extra spoiler: the next ending also isn’t the end, there’s a lot more. I will admit that playing the second run is a big grindy at first, but it quickly differentiates itself from your first run by the time you get to the first boss. Also, in the second run, the side quests are crucial.
An extra-extra spoiler: there’s a lot to dig your teeth into, philosophically, that makes the whole rigmarole worth it.
Ooh, yeah, Talos Principle! Perfect rec for someone who loved Nier and Portal. I haven’t played the sequel yet, but really really enjoyed the first game. I agree about some of the puzzles, though.