Also, I recently had an idea of an alternative team member. Assault Vest Tapu Bulu over Toxapex. Here's the set:
Tapu Bulu
Item: Assault Vest
Nature: Adamant
Ability: Grassy Surge
EVs: 252 hp, 164 atk, 92 sp.def
Wood Hammer
Horn Leech
Superpower
Nature's Madness
For those curious, the EV spread on this Tapu Bulu makes Tapu Bulu bulky enough to survive 3 Psychics from Modest Choice Specs Tapu Lele after Grassy Terrain recovery while making its physical attacks as powerful as possible with the rest of its EVs. My thought process behind this possible new team member was weighted by pros and cons.
Toxapex:
Cons-
more passive than Tapu Bulu
has bad matchup against the Tapus (which is significant because they're quite common) except for some variants of Tapu Bulu
Pros-
Is a better defensive pokemon overall
has better recovery
has a good matchup against most of the other team members' checks
can take on many of the most powerful special attackers in the tier (like Ash-Greninja, Blacephalon not carrying Ghostium Z, etc.)
can phaze
heals itself by switching out (contributing to better longevity)
can switch into fighting attacks for Kartana and Gyarados post-Mega, fairy attacks for Latios and Gyarados post-Mega, water attacks for Gliscor and Heatran and ice attacks for Gliscor and Latios
can potentially allow other team members to stall out any would-be checks
doesn't lower the power of team members' move just by existing
Tapu Bulu:
Cons-
not as good defensively as Toxapex
relies on dealing significant damage to opponents to heal effectively
sets up a terrain that lowers the power of some teammate's moves
doesn't have the added ability to phaze
Pros-
not as passive as Toxapex
can also switch into many of the other team members' checks
counters Ash-Greninja (which is a very big threat in general, but also to the team) and Keldeo
has a good matchup against all of the Tapus (which, again is very good since they're very common) except for opposing Tapu Bulu
sets up Grassy Terrain that gives Heatran additional recovery, reduced damage from ground-type attacks, and boosts the power of Kartana's Leaf Blade
takes on bulky water types that Mega Gyarados has some issues with
can switch into electric attacks for Gyarados, fighting attacks for Kartana, ground attacks for Heatran, water attacks for Gliscor, and dragon and dark attacks for Latios
So, the point of this comment is, which one should I pick? I know that the original question was about the whole team, but I would like to know which would be the better choice. I've weighed the pros and cons of each and can't come to a decision myself. Thank you for your feedback! (once you give it that is)