OK, I got curious about the efficiency of running 6 1-crown missions vs. 1 6-crown mission.

I went back after I finished the event prizes and tried the 6-crown Golem Stronghold (last mission available). It gave 3 black lightnings for completion (27 EPs, works out to 4.5 EPs per crown). 1-crown Claw Island gives about 4 EPs, but I've seen it vary. Merging 5 makes the 1-crown mission way more efficient than the 6-crown mission.

Merging the subquest pod rewards gave a single pink lightning (this varies per pod, but I think you'd do better to get six bites at that particular apple. I was surprised that the top-tier pod didn't give a black lightning or at least guaranteed multiple pinks). Remember that these do nothing for you once you no longer need EPs.

Of note...the 6 crown mission gave ZERO thunder nymph shells. It is RNG, but I've done really well collecting event-specific shells from Claw Island over multiple events and I was disappointed in the lack of payout for the increased cost and effort required.

And finally, running Claw Island six times takes about four minutes. Running the Stronghold once takes (me, anyway) about 30 minutes to get all pods and finish.