I believe you just hit upon what is called The Problem of Other Minds in philosophical terms

Do all of us experience ennui for that matter? Envy, to the same level as one another?

As noted elsewhere, this is an ongoing philosophical discussion called The Problem of Other Minds. I'd link it, but since you can't be bothered to read the links already present, I don't think there's much point.

Which leads to a paradox of how one defines a conscious, human mind at all, if it were indeed based only on what emotions are present when presented with a similar stimulus.

You're missing the point that all humanity, collectively, as a species has largely the same senses, evolutionary history, and brain structure. Therefore, despite experiencing the emotions differently and to different extremes, we are mostly capable of experiencing the same emotions. Take away that shared brain structure and shared evolutionary history, and it's a very large, unfounded assumption to think that other species have the same emotions.

Further, I’m noticing that you’re focused on dancing around “are they human”, not “are they conscious”

No, I literally agreed with you that consciousness is a spectrum and that most life falls somewhere on that spectrum. Buy hey, go ahead and ignore that so you can build yourself a strawman. I never said anywhere that I eat meat, so you're just imagining things so you can build an argument against a statement I never made.

Do you think animals are capable of being curious, even when there’s no impetus for them to be? I certainly do.

This sentence right here is everything I need to know about your stance. You're either not willing to consider or able to understand that different species experience consciousness and emotion as an evolved trait, and when the evolutionary drivers are different, the emotions are different. Any species that evolves the ability to be curious will have done so because it's an evolutionary advantage, but if the evolutionary pressure and the senses and the literal brain structure is different, then the emotion of "curiosity" will be different. Assuming that other species experience curiosity the same way as humans is exceptionally close-minded.

You're not doing other species any favors by anthropomorphizing them; you're just limiting your own understanding.

Fair. I could have been more accurate by saying "they are exhibiting behavior that has been reinforced by certain positive responses," but that's a little wordy.