If your heart is set on using the existing 30a wiring and breaker, set it up as a 14-30 outlet (or L14-30 round). Put a plug or adapter on the EVSE to match. That way nothing higher than 30a can be plugged into it.
Although I'd just redo it now to 14-50R to give the most safety and possibilities. Remove the old 30a breaker, replace with 50a breaker, use 6 gauge wires. Total parts are usually under $100. They make all sorts of adapters for 14-50 outlets since they are used for RVs.
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