So to confirm, the expectation from the taiko community (and how it works on stable) is that if you have a 4:3 or 5:4 aspect ratio, you're actually going to see less of the playfield? Basically making the window adjustment a cheap and incorrect alternative to a "fade in" mod akin to osu!mania's?

Correct.

I wonder how receptive the community would be to adding this as a FadeIn mod instead of this direction.

Probably not well, the point of this was to remove the mod dependency, not move it around.
Important to note is that people don't play 4:3 for the "extra challenge", it's just a question of comfort, and their default way to play, which would make locking it behind a mod unwanted.

Important to note is that people don't play 4:3 for the "extra challenge", it's just a question of comfort, and their default way to play, which would make locking it behind a mod unwanted.

I agree. Locking expected functionality behind a mod would likely come with criticism. That said, I don't think taiko players would be adverse to a "Fade In" mod in addition to this if it added even a slight multiplier advantage. osu!mania's Fade-In, Hidden, Flashlight combo generally seems well-liked and could apply to osu!taiko as well. I'll poll a small portion of the taiko tournament community and open a separate discussion if necessary. Unfortunately, I think this is unlikely to gain traction until #17265 is resolved.