So since we're adding submarines...

[Sairys]

Gardening = 1 person
Aether Wheel Stand = 1 person
Remodeling = 1 person
Diadem = 2 people to enter, and I believe the 2nd person can leave after you get there.
FC Crafting = Arbitrary 4 person minimum to progress. (And as stated, the other 3 people don't need to do a thing.) It is quite literally a 1 person activity, with an arbitrary added 3 to sit in the workshop.

Yes, you can pay people to sit there. Yes, you can "get more members."

The comparison to gardening and remodeling is false equivalence given that it's also an activity available through private housing while airships are not. The aetherial wheel is minor enough that it's not actually super relevant. Which then makes Diadem an exception to the 4 person thing.

-snip-

So make it more collaborative, with a reward if your fc does well in whatever that is and a detriment if they flub.