That may be, but there is a possibility that there won't be enough Ultra Aura to fully heal Lunala afterwards. Because I believe that Ultra Aura is the key to the power of a Z Move. That's probably why they can only use a Z Move one time in a battle, as it takes a while for the Z Ring to recharge. The strength of the bond between trainer and pokémon, and whether they are going all out determines how much Ultra Aura the Z Rings draw out. This may also explain how Ash and Pikachu are able to use the 10,000,000 volt thunderbolt one time, and they use Gigavolt Havoc another time. When they aren't going all out in a battle, or they are just showing off, the concentration of Ultra Aura won't be that great, and they will use Gigavolt Havoc. However, when they are pushed to their limit, the concentration of Ultra Aura will be so great, that the 10,000,000 volt Thunderbolt will be unlocked.
This may also explain why the first Electrium Z crystal shattered. When Ash and Pikachu pulled off the Gigavolt Havoc, the ring drew in more Ultra Aura than the crystal can take, thereby shattering after the first use. This also explains what happened in episode 19. When Ash and Pikachu were going all out on that large hamster wheel, the ring started drawing in Ultra Aura, causing an explosion. And when Ash was battling Tapu Koko, when they had their backs to the wall, the Z ring started glowing again and drawing in Ultra Aura. The only reason that the Electrium Z crystal didn't change into the Pikashunium Z crystal was probably because the original Z ring didn't have the power to do so.