I have been thinking about this (probably too much) over the last couple months since I got my Pros. There is a solution that I just figured out today. It’s not anywhere near ideal or necessarily practical for most people, but you can turn off your AirPods while in the case and stop them from charging further.
To achieve this, you have to factory reset the AirPods/case. Press and hold the button on the back of the case for about 15 seconds. The light on the front will blink white for a bit, then shine amber for a couple seconds, then switch back to blinking white. Once the amber switches to white, you’re done. Your AirPods are now turned off and will not charge again (or experience noticeable battery drain) until you again pair them to a phone/watch/whatever.
It’s cumbersome for sure, but I’m gonna try doing this every night (and especially on the weekends, when I don’t use them nearly as often) for a bit and see how I feel. I believe this is the only option, and I think it could probably at least double the shelf life of the AirPods. And of course it works best if you stop charging around 50-70% to minimize damage to the battery while off.
Of course, Apple could easily create an option to toggle the AirPods being able to charge/not charge while in the case. I mean, they won’t do that—but they could.