> Methane breaks down to Co2 in the atmosphere on average in 9.6 years.

Despite which, over a 100 year period it has 25 times the warming impact as the CO2 it turns into.

> The problem we should be focusing on is fossil fuels.

Everyone should focus on whatever they know how to make a dent with, the "sustainable" emissions rate is 0.1% of the current rate which is essentially "we need to fix everything everywhere except for pick one of grassland or North Korea".

https://ourworldindata.org/ghg-emissions-by-sector

Animal-emitted CO2 isn't the problem, but methane.

The CO2 captured by plants and eaten by livestock gets converted in part to methane, a more more potent greenhouse gas. This does eventually degrade back to the original CO2, but on a scale of decades, it's currently too slow to prevent accumulation of methane.