I think any vegan who is at least somewhat sane would say squishing an ant is not the same as killing a cow, because one is clearly more sentient and capable of experiencing suffering than the other. Obviously you don't want to do either of them, and yes overpopulation and food waste causes us to kill enormous amounts of insects and wildlife. But I also don't advocate for increasing the human population and try to waste absolutely 0 food because I know the impact it has. Buying food locally from non-factory farms is also much better, vegan or not.

It's also true that animals consume and require MUCH more resources than just eating the plants ourselves, so even under that framing we are still doing significantly more harm because now we have to grow extra food, have space for these animals, and deal with the extra emissions from feeding/giving water/transport.

I absolutely can't support myself nor anyone else with some backyard gardening, that's definitely true. But with 2 people doing it in our leisure time, we reduced how often we have to buy any foods by about 40% by having ~8 fruit trees, 10 raised beds, and a freezer. Obviously this isn't 0 harm because we do need a fence and sometimes have to trap and release some animals, but it is possible and not infeasable to feed a neighborhood by gardening like this, and it's been done in many neighborhoods already.

You definitely can not exist without causing harm, and meat is not the ONLY negative thing to reduce, it's just the most obvious and requires the least amount of legislature to actually reduce the harm from it. If we had a much smaller amount of cows that only ate the waste, that were humanely slaughtered, I think most vegans still be sad about it but not be nearly as upset, and meat would be a special thing rather than a staple.