So while the current design works for the fighter, where the expertise dice feed directly into combat maneuvers, we can see it breaks down for something like the monk, where some of the maneuvers are more "set up" maneuvers.
Perhaps it would be better if the monks maneuvers all used ki, and what the monks combat dice represented was the amount of ki available on a given turn. So for example at the start (or end) of the monks turn, they would roll their expertise dice, and this number represents the amount of ki they have for this turn to spend on their maneuvers. You could then allow ki to bestowed across cumulative turns, but only up to the max value of the monks expertise dice.
For the thief it's a bit harder, but maybe instead of applying to combat at all, expertise dice should be how the thieves "skill monkey" trait manifests itself. The thieves expertise dice canbe used to add to their skill rolls and ability checks in a given turn, or alternatively if they're "aiding another" they can spend those dice to aid someone else's skill or ability check.
If they use expertise dice like this, I could see them being able to stretch the core idea across multiple martial classes, where each classes uniqueness comes from how the expertise dice contribute to the class concept, rather than being 20 different forms of the same basic combat maneuver.
I am continually impressed that folks on this forum generate more interesting and thought provoking ideas and refinements than the designers at WotC. This applies to quite a few more posts than the one I've quoted, this was just one of the more recent ones.
Kudos to everyone. And, thank you for giving me some cool things to enjoy and possibly incorporate into my own games.
Frolic and dance for joy often.
Be determined in your ventures.