This is actually why there have existed fruit-based diets and entire religions and other ethically-minded lifestyles based around the idea of minimizing suffering, for example eating root vegetables or other techniques which require killing plants outright.

I do find contention with your idea that animal-based diets can be on par in terms of resources needed per calorie produced. Of course it is not easy to obtain 2000 kcal from lettuce or something with similar low levels of calories versus the farming inputs, but things like beans and other high-calorie foodstuffs are obviously vegan and have much less ecological impact than all of the feed inputs that cows, pigs, or even chickens require per kcal.