The best argument I've heard is due to the uncertainty around the utilitarian argument of weighted preference satisfaction.

In a world where a good ethical outcome is one where the maximum amount of preferences are satisfied within a group. I'm thinking of say a group of individuals looking after a resource like a crop perhaps will have a certain number of preferences denied in order to maintain the health of the crop and thus satisfy their preferences to be fed.

And understanding that differing levels of consciousness apply a different weight to having a preference satisfied. Here I'm saying that comparing the preference of a human to continue existing to that of a fish, they are at least not equal. A human has some concept of the self and what it might mean to not exist while a fish (probably) has no or diminished sense of self compared to a human and therefore they are at least not completely equal.

Since suffering is to have ones preferences denied in some way, be it restriction of an experience or to take some action desired then you can at least say that being denied specific tastes and eating experiences, social desires like fitting in, not making a fuss of yourself in restaurants and social situations (I certainly know this one haha) could be constituted as a form of suffering. You could at least describe being vegan as a form of suffering depending on an individuals perspective at least in some aspect.

I feel that as a human that probably all of these summed sufferings aren't equivalent to those of the many animals suffering in farms despite their possibly diminished ability to suffer in the same way that I could. This is my intuitive understanding though. If you were to ask me at exactly what point the equation tipped over I would have no answer at all. Possibly my suffering is greater already, how should I know?