I imagime that the capstan and pump shanties want to be very rythymical (and not too fast).
The timing of the long and short haul shanties can be a bit more varied, as if the shantyman is waiting to see that the haul is ready.
I'm pleased that you are making as much effort as you can to get things right, Neil. Not many people bother. Hopefully someone with practical experience (Cranky Yankee?) will be here with some advice as to beats per minute. My own opinion is that shanties sung as songs are sung too fast, usually.