Welcome back! I am also currently working on a 75 gallon display vivarium:
And I have a different 75 gallon on the back burner as well:
Of all the frogs I've kept so far, leucomelas would be my choice for a large display vivarium. They are bold, colorful, do well in groups, and use all of the space.
Phyllobates terribilis is another good display frog, but I personally prefer something with more than one color and or a pattern. I expected P. aurotaenia to be bold but that hasn't been the case for me.