With this Xurkitree, we are almost at full prize coverage and type coverage. Beast tool at 6, baby buzz at 4 for them, xurkitree at 3 for them, nihilego at 2 for them, nag at 3 for us, kartana (forget what prize count there). But an ultra beast tool box could totally be a thing.

Depends on how you look at it. There are still several holes since the non-GX Ultra Beasts also differ on whether the take your prize card count, or the opponent's prize card count into account. Here is a quick list:

Buzzwole (Forbidden Light): Extra damage when your opponent has taken exactly two prize cards.

Xurkatree (Forbidden Light): Instant paralyzis when you have exactly three prize cards remaining.

Xurkatree (Sky Legend): Extra damage when your opponent has taken exactly three prize cards.

Kartana (Celestial Storm): Extra damage if your opponent has exactly six prize cards remaining.

Kartana (Full Metal Wall): Extra damage if you have exactly four prize cards remaining.

Celesteela (Celestial Storm): Attack cost reduction if you and your opponent in total have exactly six prize cards remaining.

Dawn Wings Necrozma (SM Promo): Extra damage for each damage counter on Dawn Wings Necrozma if you have exactly six prize cards remaining.

Dusk Mane Necrozma (SM Promo): Extra damage if your opponent has exactly one prize card remaining.

Naganadel (Lost Thunder): Extra damage if you have exactly three prize cards remaining.

Nihilego (Lost Thunder): Copy one of your opponent's attacks oin the battlefield if your opponent has exactly two prize cards remaining.

Pheromosa (Forbidden Light): Extra damage if you have exactly one prize card remaining.

Stakataka (Full Metal Wall): Extra HP if your opponent has three or less prize cards remaining.

Ultra Necrozma (SM Promo): Extra damage if you and your opponent in total have six or less prize cards remaining.

Guzzlord (Forbidden Light): Discards ten cards from your deck if you have 2,4, or 6 prize cards remaining.

Blacephalon (Unbroken Bounds): Extra damage if the flipped over prize card is an [R] energy card.

Xurkatree from Sky Legend, Buuzwole, Kartana from Full Metal Wall, and Nihilego seem to work best in either Ultra Beast decks, or as tech cards in Electric decks. Personally I believe that you get a great and sufficient coverage with Xurkatree from Sky Legend and Nihilego. Note that both can be used with two to three tech Unit Energy from the same type.