While we can’t be completely sure, our current understanding of sentience makes it a reasonable assumption. Even if plants are sentient, eating from higher trophic levels causes more plant deaths than eating plants directly.
Regarding the rest, I feel like I addressed all of that in the comment above. I’m a fallible human being and personal discomfort with killing animals no less cognitively complex than our pets, and sometimes toddlers, is definitely a factor, but I’ve been arguing based on necessity and quantity instead of that.
EDIT: And to be clear, I’ve never claimed veganism is environmentally perfect. It doesn’t solve every problem with food production, it just helps with some, and it seems largely better for the environment (albeit with nuance around grazing certain types of land) even if we keep doing monocultures.