If I wanted to make a team with a core of Z-Kommo-o, AV Magearna, and Mega zam what would be the more important pokes to prepare for?
Ash-Gren: you're relatively good on this front but you want some answer to spikes
Heatran: again, good, but stally status variants or Z movers can take out Kommo or Zam if played well - probably not both
Rotom-W + Zapdos: you're actually pretty good against these two since Kommo can nuke with Kommonium. Just be prepared for them to try to mind-game you by swapping out when you hit that Z attack.
Prot-Gren: you're ok on this front, Mage can get worn down by HP Fire
Tapu-Koko: you probably want something that can eat several Scarf Tbolts or cancel Electric terrain, like Bulu
Rockium/Scarf Garchomp: you're weak here. Chomp can threaten all of your current mons from full
Landorus-T: you have relatively nothing on Scarf Variants and could get dismantled by a SD variant
Magearna: none of your mons can beat CM+SG once it gets rolling, AV can just dance around you with Volt-Switch and can hard check both Kommo and Zam
Medicham, Mawile, Kart: all of these mons have strong, fast attacks and can threaten you. Lele can help to ease the threat of Fake Out but won't provide any help vs. Kart
KyuB: a threat to nearly every team that can be prepared for but not always beaten. AV Mage can eat a couple attacks but will eventually break. Since KyuB outspeeds neutral Kommo and Scarf RKs, your opponent can mindgame you. LO sets with Tapu Koko are your biggest weakness rn.
Volcarona: also a threat to every team that can be prepped for but not always beaten. While Kommo can check it, it's not a foolproof counter.
Tyranitar: while Magearna and Kommo-o resist this mon, it's strong enough to break you with coverage. Band variants are scary, as are DD and Rockers, either of whom might trap kill M-Zam. Sand itself is a threat given that M-Garchomp + Excadrill can break many common cores and none of your mons explicitly check them outside of Zam tracing SR after Exca nets a kill.
Stall: None of your mons are particularly good against stall, as Unaware Clef/Quag will hard-check Kommo-o, Magearna+Zam will be useless against Chansey, and M-Sableye can threaten you with Wisp or Knock Off to some degree.
There are a lot more mons, but these are the big things to consider when building a team, IMO.