It's tough but I like Fires for the flexibility.
I love Recoil. It's the best example in Magic of the sum being greater than the parts.
Selesnya Charm's make a 2/2 dude is a nice way to kill an unsuspecting attacking morph actually, and once unmorphed, I think there'll be a fair amount of 5+ power targets? Most of them will be green though, so there's that. Dromoka's Command lets your morph kill their morph (+1/+1 counter & fight), and is a bit more verstatile.
Did you consider any of these?
I think all of those are more interesting than a second Battlesphere or a first Sundering Titan.
360-card cube can be made efficiently, but you would lose the element of brewing with non-optimal cards. I don't want that. My gaming group(s) don't want that.
I personally have a 450-card cube, and I would say it definitely is not super efficient, and it definitely still has room for darlings, and not all of the pieces show up all of the time. It feels a lot tighter though than a 720 I would bet. But, well, it was just a suggestion to attack your problem from another angle. Do with it what you will