Yup, the pace and plot lines are nice.
Now we know why the village rules are so strict.

Regarding folklore, particularly the maiden sacrifices to the sea God. I wonder if it was really a sacrifice. Maybe those girls were in fact given the ability to breathe under water. But the condition was to never set foot on land again. So for surface people they were as good as dead, explaining the sacrifice naming.
That would be a practice giving more females to the village so that its population can grow.

Last but not least, I did not expect the geezer to be a former village guy
It explains why Tsumugu knows so much.
Back to the folklore idea... I wonder if Tsumugu would be compatible with the process giving him ENA.
That would be nice tom have Akari choosing to go to the surface, the geezer who did the same and Tsumugu choosimg the village. So that it's not just some mysogynic custom and show choice.