Fantasy Series Idea is Disjointed

I’ve been making my outline and planning for a 12 volume series and, without getting into the plot, I’ll get to the issue at hand: In each volume my main characters go to a new place separated from each other in a kind of archipelago country. The issue I’m having is that I don’t want to run into ‘planet of hats’ or ‘island-hopping’ problems, but I’m not sure how to make one volume transition to the next. And, in addition, would the last volume be in its own new place or an old place they already visited? (And would this be weird since every other place is visited once in each volume)? I am seeking advice on how to do this constructively, rather than criticisms on series being hard or my experience level. Many thanks! :)

EDIT: Maybe I should’ve mentioned this is intended to be a graphic novel series, so 12 volumes isn’t as much heavy lifting (well it is, but the art does a lot of the work) but I guess I’m having difficulty either blending locations together or having each location be interesting and unique that putting more than one in a volume seems like too much