you are correct, there is no thunder healers currently in game, either here, or in DMM, so if you decide to go full thunder, then either marry Andromeda for life, or pull some sort of healer of another element.
in the future, (Ixion event, which is not too far now) we will be gettin 1 guaranteed kamihime weapon gacha ticket per event, so that might help, other then that, in DMM there were couple of sales that sold guaranteed SSR hime ticket for equivalent of 50$ each, but thats for whales. When it comes to free players, its all about the luck, thats how this game works =_=