Do caster out of combat options matter?

[Psyren]

Make a party of all strength based fighters. You will not be able to defeat the dragon unless you are playing on explorer mode. They'll also REALLY struggle against Kethric Thorm, I would expect an honor mode to struggle to enter act 3 with all strength based GWM PAM fighters. The magma elemental will also be unbelievably tough (in honor mode it creates its own magma, so you need someone who can cancel out its regen).

We're talking about plot and narrative here, not video game difficulty. Honor Mode is irrelevant.

There are certain outcomes with Kagha that are virtually impossible to get without having a druid (preferably, a high intelligence one).

No no, put those goalposts back down. You can resolve the Shadow Druids plotline regardless of party makeup; that's what matters. "Certain outcomes" is not the question being asked.

There's the spiders in the goblin camp, you can get a certain outcome if you can talk to them. Relying on potions is IMO not reliable, there are more animals to talk to than potions. In fact having speak with animals in BG3 makes a massive difference.

The plot there is "defeat the goblin leaders." Whether the spiders can help you or not is irrelevant, they are one small way of getting help with that plotline that isn't even needed.

There's an outcome with Auntie Ethel that requires someone in the party is good at charisma checks and/or can boost ability checks.

Same as above; "one outcome" or "certain outcomes" does not matter, you can resolve the Auntie Ethel story regardless of party makeup.

I'm sure there is more, but I'm equally sure it's not worth the time and energy to find them for the sake of this thread. (also, yes I know about save scumming, and no I don't consider it valid, you'd never be able to save scum at table top)

You haven't found any, nor will you.