After months of struggling with hardly any volume in my right bud, I tried this and it worked. I reasoned that what I really wanted was to start again as if they were fresh out of the box. Once I set that as my goal, the rest was obvious. So....
1) Go to the wearables app on my phone and press the reset. I tried that for months on and off and it had no affect. In my experience this only resets to your default settings. It in no way addresses the sound imbalance between the two buds. But definately do it now.
2) Stick the buds in your ears and set about completely draining all 3 batteries. You can watch them drain on your app. Be patient and be thorough. Once all of the power has been drained out of them, that's the closest they're going to be to being box-fresh.
3) Put your buds back in the box and give them, and the case, a full charge.
My problem was much improved after this, but it was not perfect, so I repeated the above one more time. They are now fully restored to their former glory. If any problems arise in the future it's straightforward to repeat.