The calculations for feeders are 100% of noncontinuous loads + 125% of continuous loads. So the 24A times 1.25 is 30 + 12 = 42. That would need at least a 42A supply to the subpanel, eh?
Normally, I wouldn't trust myself to do a more complicated load calculation on a subpanel if it is considered living space, and it has several circuits, where you have to apply some amount of watts per square foot and all of that, but this one is really simple because the 24A continuous supply to an EVSE really does consume the entirety of a 30A supply circuit, so there is zero capacity leftover for anything else at all.
^^^^^
This.
Exactly correct.
But OP says there is just a garage door opener and a few LED bulbs, so in the real world putting in a 30 amp breaker, and charging at 24 amps will cause no problems. Running a hand tool, like an electric drill, due to its short period of operation would not cause a problem. Maybe running a 12 amp vacuum cleaner for a while would be a problem.
That is why the NEC would consider this a violation.
If it were me, I would risk the 30 amp breaker. The worst case is you might pop the 30 amp breaker in the main panel. If it became a problem you could downsize to a 20 amp setup.
While a 20 amp circuit will very likely be adequate for the OP's needs, there are reasons for going as high as you can. Here are three:
1. the rare time you might come home with a low % and then need to add 10 or 20% because you need to go out soon thereafter.
2. Charging at 24 amps is more efficient than charging at 16 amps, because the car is awake longer at 16 amps.
3. If the OP has time of day billing, he may be limited in the time period he can most economically charge.
Later, if you do put in a 30 amp circuit, if you sell the house and the inspection says this is a problem, or if anything else comes up, you can always put in a 20 amp breaker.
I do question the OP's ability do do this himself since he seems to have very limited experience. But hopefully everything here will help him when he speaks with his electrician.
OP, I wonder what you ended up doing? Would you let the community know?