I have definitely thought of going the 'interstitial' ad route (show one ad only at the end of every N rounds). But the reason the game is so addictive, is at the end of a round, the scores are shown for all players playing (hence the need for a Cloud connection) and then the next round starts without any user interaction. This is key.

There are no higher game levels. Each round is different.

Can't do a premium version, since there are ongoing server costs. I want those covered by advertising. I don't want to have to fund the servers from now till forever with potentially no future income.