Are you required to report leaving an altitude if you have been given a crossing restriction?
I’ve been a controller in both the en route and terminal environments. While I agree with your interpretation of the AIM that it appears to be required, reorting leaving your altitude is not expected and I did not usually hear it done.

To give the perspective from the other side of the mic, an altitude crossing restriction is functionally equivalent to a pilot’s discretion climb/descent to that point. ATC will protect all altitudes between your current one and the restriction, and you are free to hit that restriction as you see fit (barring the violation of other regulations, of course). You may not return to a previously vacated altitude as part of your climb/descent, but you may level.

You are also correct in that it can tie up the freq as an “unnecessary” call, but if you are unsure, better to over inform rather than under inform.

Incidentally, very few controllers read the AIM. The 7110.65 takes precedence, and I’ve actually had a supervisor yell at me for referring to the AIM once. I know, I know.

So, legally, it should be done.  In practice? Unnecessary and I preferred that it not be done when I was working.