This whole "have one fewer child" thing is totally bonkers, because even on the face of it, it really only makes sense for people in Western nations with their current lifestyles. It's also an average over all the people in that country, meaning it's heavily spoiled by rich kids. Essentially, 1. you can't know beforehand how your child will live and 2. emissions don't scale linearly with the number of people (again, look at the difference between countries). And then there's the anti-humane undertone of it.
I don't think that we should focus on something specific, for sure you will not convince everyone to go vegan, but it's the same to go car free
Some people might find it easier to to vegan and stop plain that not owning a car, and other opposite, I think that at this point everything is good to take
The average trip length in America is something like 2 mi in distance. That's a distance that you could walk, and you can bike that in less than 5 minutes. So Americans really can meet a lot of their daily travel needs to the store and short errands by means other than a car.
The biggest problem in America is twofold: infrastructure and behavioral patterns.
Your point is valid, but the fact is none of those are enough on their own. Even if we get rid of all emissions except for the cattle industry, wed still shoot way past the 1.5° mark. So not going at least vegetarian was never an option.
The environmentally beneficial effects of plant based diets or a vegan lifestyle are not reduced to harmful GHG emissions alone but encompass a wide range of advantages. To name some:
Reduced agricultural land use (the vast majority of land is used to grow cattle feed). This can also reduce deforestation (especially interesting in the Amazon region), increase ground water and soil quality. Avoids soil erosion. It also perserves eco systems on land and helps to mitigate species extinction.
Water usage. It takes about 1000x to produce meat than to produce an equivalent amount of, e.g., wheat.
Reduction of overfishing and thereby protecting and stabilizing oceanic eco system.
Reduction of the huge amount of water and air pollution caused by the animal industry.
In general sure, producing less and consuming less leads to less impacts. But there are quite the differences in what and how we consume it with regard to their impacts. For example, we don't need agricultural space for mining cobalt to build batteries which power electric cars.