Some alternative ideas:
1. The slash in the grace note has a different meaning than the slashes in the "off" commands, which might be confusing. In the "off" commands, maybe use a slash along with a gray color? (Inspired by European "end of speed zone" signs.)
Attachment:
gray-off.png [ 1.23 KiB | Viewed 439 times ]
2. You skipped
p and
f. Some people might wonder where they are. Maybe just using a single volume symbol (like a triangular ramp shape) along with a number would be easier to understand, and allow more than 4 levels.
3. The note stems imply a specific note duration. You might use a filled notehead without a stem as a way to represent a note with no specific duration, but maybe that's more confusing.
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no-stems.png [ 966 Bytes | Viewed 439 times ]