You seem to be using tap rules to justify undersizing the egc.
Without getting into the issue of 'are taps allowed in this circuit', _if_ a tap were to be allowed I could see your point that the egc could be reduced in size to match the size of the taps.
For example, if one had a 50A cooking appliance circuit feeding a 20A cooktop and a 30A oven, I could see your justification used to run a 12ga EGC to the cooktop along with the 12ga supply conductors.
But in this case, we don't have a tap. We have the full 10ga 30A supply conductors feeding a receptacle. The egc must be sized for the 30A circuit, as per table 250.122.
If you look at Table 250.122 you will see the following wording.
Quote
Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)
There is no code section that allows reducing the EGC run with tap conductors other than 250.122(A) which tells us the EGC is not required to be larger than the ungrounded conductors.
Bob Badger Construction & Maintenance Electrician Massachusetts
