2nd para beneath the side heading: "Are there alternatives to the 100-year GWP for comparing GHGs?"
GWP calculations has been an active area of research for a long time.
For instance, if you look up the table beneath the VALUES side-heading in https://en.wikipedia.org/wiki/Global_warming_potential, you would notice that from 1995IPCC to the recent one, Scientists are getting more and more closer to ~80 GWP.
So, redrawing your path by your method, if we remove ALL of the Atmospheric Methane today, we would be reducing GHG concentrations by about 160ppm CO2 equivalent, which takes the overall CO2 concentrations to less than that of pre-industrial levels(280ppm), 260ppm.
This negates your conclusion that we would hardly observe ANY difference.
If we remove literally all of the Methane today, we would have solved Global warming from the perspective of concentration of Green House gases and will just have to wait and watch for the Global Temperatures to catch up (meaning they will go down).