Look at your assumptions: That suffering is a good measurement to use to draw the line, that large animal suffering is the place to argue about it, and that large animals yes/no is the granularity to use, and repeatedly coming back to whether I can demonstrate that plants feel suffering, despite my claimed disinterest in that point.
All of the above are arguable - why not argue that humans and cattle are inedible, but question where the line between insects and small mammal suffering is? Why not question whether stupid non-suffering cows are OK to kill? Or whether non-suffering humans are OK to kill and eat?
Even if you continue with your line, there are other ways you could argue it - if suffering is bad, then that doesn't mean eating animals is bad, you could tranquillise before killing and it would be OK. Or if you want to minimise suffering then wild animals aren't OK, and all animals should have a human managed lifespan from sedated birth to anaesthetised death followed by being eaten, and not-eating-animals but leaving them to die "naturally" is worse.
And there's room to step back and say suffering is a behaviour and thought pattern that's only of interest to the creature connected to that nervous system - otherwise of no interest, so why go for 'minimise suffering' at all? You could argue about methane emissions or water/other resource use, for why it's unethical to prioritise farming animals for food, or you could look at it as a social signalling - wanting to signal that we understand suffering and choose to avoid it or as a personal suffering (guilt) reduction aside from the animal suffering, or you could believe in a deity which punishes you after death for animal suffering you caused.
But however you look at it, you come back to: humans have to eat, so plants or animals have to be ethically OK to eat, and we don't want hurt selves or relatives and friends, so cannibalism can't be OK, but within that, whether extending compassion to animals is "right" or not, it's not objective - refusing to eat farm animals because the calculation on their methane emissions leading to global warming effects and future human suffering would be, but where the ethical line in farming living creatures for food is, is not. It's just arguable back and forth.
I eagerly await your finding that vegetables feel pain.
Even if they don't feel pain, they have a natural life, lifespan, life processes and reproduction which eating them stops. There is room for an ethos where doing that is wrong, you just dismissed it as daft.