Played through the entire game and beaten it. And I can share a bit of information.

Seven Trial Captains in total: Normal, Grass, Fire, Water, Electric, Ghost, and Dragon.

The four Kahuna specialize in Fighting, Rock, Dark, and Ground types respectively.

Each of the four Kahuna upon defeating them will grant you the keys to access the ruins where the Guardian Deities are hidden. However, you'll need the necessary Poke Rides in order to access them.

The "eighth trial", in a sense, is to capture the four Guardian Deities, which will award you with the Guardian-Z item for capturing them. Afterwards, you will need to battle the final Totem Pokémon--Solgaleo in Pokémon Sun, and Lunala in Pokémon Moon. They are both the same sizes as their original counterparts, though they will always open up with their respective Z-moves in order to enter their Radiant Sun and Full Moon phases. You cannot capture them in the initial confrontation, however, as you must defeat them (it pretty much goes without saying, but you cannot capture Totem Pokémon).

The reward is the Psychic-type Z-Crystal, and an opportunity to face against the Island Trial Champion, Kahili.

In addition to Solgaleo and Lunala, other Pokémon have their own unique Z-moves that are tied to one of their signature moves. For example, Alolan Raichu can learn Surf, and the Electrium-Z in conjunction with the move is how he learns his unique Z-move. Same thing applies to the Starters with their respective signature moves.

You get Cosmog from Lillie in the post-game and that is how you can evolve it into either a Solgaleo (Sun) or Lunala (Moon) of your very own.

I'll answer any more questions you might have concerning the game itself. Still working on the postgame at large--mainly trying to locate the missing Z-Crystals, but I'm also playing through the postgame.