So is there any reason not to point this at Proxima Centauri b, like, ASAP?
"Over the coming year, researchers will use spectroscopy to analyze the surfaces and atmospheres of several dozen exoplanets, from small rocky planets to gas- and ice-rich giants. Nearly one-quarter of Webb’s Cycle 1 observation time is allocated to studying exoplanets and the materials that form them." - https://www.nasa.gov/image-feature/goddard/2022/nasa-s-webb-...
An obvious target for the coronagraph for regular imaging, but there's no way to get a transmission spectrum of its atmosphere.
The real problem is that Prox c. b is only 0.04 AU out from its host star. So the absorption spectra for a star lined up with the planet is going to be pretty well contaminated with light from Prox c. You could imagine various schemes for moving around the observer for the best angle or big occultation disks, but at a certain point it's going to be easier to just fly a probe over and sample the atmosphere directly.