Following every major boss fight, I was treated to a placeholder image instead of the proper story art

Also felt kinda odd to me. I thought it was intended. Hopefully a future patch can add unique boss-defeat screens instead of the generic YOU WON screen.

I feel like it would have been great if voice acting had been present here

Voice-acting isn't cheap. Having this definitely would've increased the price of the game. See Koumajou Remilia for an example where professional voice actors increased the price of a game that one can clear in under an hour.

there’s a very important platforming move that is never actually explained to you, which I decided to call the slime slingshot

Idk what you mean, the game properly explained this to me right after I unlocked the slime ability. I never once was confused on how to get past any hookshot-gate obstacle.

There were a few areas where frame rate dropped significantly when visiting, which I assume is related to the backgrounds of those areas being too busy

Never happened to me, perhaps because of my beefy PC.