It would be great if we could use the Apple Pencil Double Tap feature to do “something”. I know this was mentioned a long time ago, and at the time you mentioned it seemed to be unreliable. Perhaps Apple has made some improvements to the code, because it seems to work great now to switch drawing tools in Apple Notes or other programs. Also, they have added fine tuning of the function in the Accessibility Settings Menu. Perhaps you can reevaluate the usefullness.
I can think of several useful functions: Undo, “Done”, open/close “Items”, toggle “hidden lines”, etc. Ideally, the desired action could be user selectable from a list of choices. In a perfect world, each user could select from ANY possible function. Since we all seem to have our own preferred work flows, that might go a long way towards removing pain points for a variety of different user styles.
It might even be useful to have a user-configureable mini-menu pop up (similar to a right-click on Mac), although personally, I think the sweet spot is one, instant (user selectable) action for really common actions. For most “normal” activities, the current “smart” menus are very good (and getting better).