Mythic rank 3!
It looks like my options are Angel and Demon, which is a little disappointing as I was looking to do Legend, but ok. Angel it is.
Abramo: Ascendant Summons - summons have been very weak against demons but if they can ignore damage resistance then everything changes!
Lann: Cleaving Shot because area-effect damage is awesome and he is frequently the killshot.
Seelah: Mythical Beast because Irony is an awesome horse and it would be nice if he could actually contribute in combat against demons. Probably not very optimal tbh.
Nenio: Ascendant Element (Fire), which is a little boring and also Nenio does more battlefield control than direct damage, but I'm just not that impressed by the other mythic feats available to a wizard.
Regill: Hmm... I'm torn between Magic Nullification which seems thematic, Thundering Blows because area-effect damage is awesome and I hate when Regill misses anything, and Last Stand which also seems appropriate for the iron-willed Hellknight who will not die until he has finished the mission. Unstoppable fits too, but there are just not that many paralyzing, nauseating, or frightening threats that Regill doesn't have a pretty good save against. In the end I come down on Magic Nullification because I like the image of Regill just no-selling any magic that would interfere with getting things done.
Sosiel: Domain Zealot because his "Luck" domain abilities are really rather good and having them as standard actions is the main limiting factor; moving that up to swift actions will be surprisingly powerful, I think.
Daeran, Ember, and Camellia: Improved Abundant Casting remains a no-brainer for primary casters, sadly.
And after a quick detour to zap a vampire, level 9!
Abramo: Cleric 9, and fifth-circle spells, ha. Feat is Spell Penetration because spell resistance is really annoying.
Lann: Demonslayer Ranger 2, because the Combat Style looks cooler than the remaining Ki powers. Improved Critical (Longbow) because a Lann critical often ends even a tough fight. Combat style is obviously Archery and first feat is Toughness, although I don't understand why Rapid Shot is disrecommended.
Seelah: Paladin 9, of course. Improved Critical (Longsword), same reasoning as Lann. Exhausted for the Mercy because that's really crippling when gotten from an enemy spell and I super need a counter for it; ask me how I know.
Nenio: Wizard 9, lest Nethys be pissed. And Deft Hands for the feat, along with putting all the skill points into Trickery, because I really need to replace Woljif for locks.
Irony: Endurance because I'm leaning into Irony being tough as nails - his horseshoes have to be attached using adamantium or mithril - and also he's my go-to for Athletics checks which is otherwise a weakness of the party.
Regill: I don't understand why the game keeps recommending Armiger over Hellknight? Perhaps it's because the lower levels of Hellknight don't have that much Cool Stuff, but then, they are prerequisites for getting to the Cool Stuff, so... Ok, fine, I'll take Armiger 8 and the bonus combat feat. Which is Improved Critical (Gnome Hooked Hammer), and then Critical Focus because why not stack bonus on top of bonus?
Sosiel: Another Cleric 9. Feat is Spell Penetration because Dismissal is really powerful against demons.
Daeran: Oracle 9, why not. Spell Penetration.
Ember: Witch 9, Greater Spell Penetration.
Camellia: Shaman 9, Elven Spirit.