Reducing methane emissions is a quick win (and "only" requires us to shift diets away from beef) which buys us more time to deal with fossil CO2 which I agree is the main problem.

Also, a lot of forest destruction is associated with beef production (often via soya) so again it's something we could get a short-term gain from and longer-term we need to reduce CO2 and forests are the only proven technology for doing that.

I think EVs are an easier sell than getting rid of beef in diets. The cars are faster, quieter, and need less maintenance. Charging is the only problem left.

In both cases the goal should be to reduce the amount. Less ICE engines and less consumption of beef. Impossible and Beyond plant based meat is very close to ground beef and largely satisfies. Electric cars will work for 90% of people in the near future. There will always be exceptions. Sometimes you need an ICE vehicle for a specific application and sometimes people will want a good steak or burger. That is okay. Reducing both by 50% would be a huge step in the right direction and would require little sacrifice.

But to a large extent the CO2 burped by cows comes from the sequestered carbon from deep underground in the form of fertilisers used to grow the food we feed to the cows.

I wonder what proportion of the carbon in a plant comes from fertilisers vs CO2 taken in while growing?

It looks like some fertilisers do contain carbon - but not all. Eg ammonium nitrate doesn't, urea does. The ammonia tends to be produced using methane - so right now the fertiliser production process does produce vast amounts of CO2 (but that isn't carbon which goes anywhere near the cow).

The CO2 from ammonia production seems solvable (assuming an abundant supply of non-fossil fuel energy) through the electrolysed hydrogen -> ammonia route. Would that be enough, or is the carbon locked up in minerals used in phosphate production also a major source of CO2?

While ammonium nitrate itself does not contain any CO2 producing it does release it as our energy system is not carbon free (yet).

https://www.fertilizerseurope.com/wp-content/uploads/2020/01...

Also you need to transport the fertiliser from the factory to the fields (diesel trucks) and spread it on the fields (diesel tractors)