Do you know Yamato's DF numbers?
I also wanna add that when we add all SHs Numbers together, we get 56 + 110 + 87 + 43 + 29 = 325
And 325 is read as San Ni Go, which clearly refers to their Ship Sunny Go
Yamato joining doesn't match the Pattern and would ruin the Equation too
(And this is only single reason why i believe something gonna interrupt them & not allow Yamato to join)