Not got loads of experience playing resonas but, given their high cost, I'd definitely recommend not using more than three. Having played banishing focused decks, I've wiped resona decks because my opponent has paid too much for them and been left with nothing after a couple of well placed abilities. Just from my personal experience :)
Sorry this took so long to answer, life got in the way.
Admittedly, I mostly play on Webxoss right now so the deck isn’t as up to date as it could be, but so far I’ve not really had any problems with too many costs late game. I usually get a nice little finish, ideally resona Saturn with Alhena in hand to then get Neptune or Venus then use them to get Uranus. Or even just using Alhena into Netune and Venus for Uranus with some off colour double crush/lancer effects with some lucky drawing and then using Resonance March defensively if it goes through to the next turn. I very much keep the resonas back for end game.
My problem is actually early game. The Space SIGNI don’t seem the have the consistency in defense, aggression or search power that many other decks have, possibly because they’re newer. So really my need for defensive ARTs is a matter of protecting myself until I can play the resonas later on.
Whoops, I kinda rambled a bit but there’s not really any quick way of putting it.
