Over those 12 years, methane "decays" to CO2 and water vapor, both greenhouse gases in their own right. And before it decays, it has a much stronger warming effect than either. So a ton of methane is strictly worse than a ton of CO2 in absolutely every way.

Sure but in those 12 years it does an incredible amount of damage(and then it decays into CO2 anyway). Methane is a huge problem.

However, it has 28x the global warming potential of CO2.

Methane breaks down into CO2.