Does that mean I would want slightly different formations for manual vs. auto solo play? And in general, are you saying I should form the backline with more regard for support skills than for helping with damage?
I do think one thing I forgot to take into consideration was that, when playing a quest manually, my lead character under player control is attacking at a much faster rate than the AI and probably "stealing" the hits that the AI needs to fill up skill gauges.
Yup. I'll create different team formations to use different playstyles; it's the thing I love most about the game.
Most of my setups will have one Support Adventurer on the team. Taking a look at Elisanne; her maxed Co-ability will increase your team's HP by 15% and her Skill 1 will increase your team's Strength by 20% for 15 seconds without unlocking her Mana Spiral. That combination of strength/HP she gives will help make your runs smoother.
If you're playing manually, you can play more aggressively since you can time your skills to protect your teammates from red attacks. You and you teammates are invincible to red attacks whenever you use a skill and your skill's attack name / adventurer photo are shown. You can setup your team with a main lead DPS + Blade/Wand for +10% Strength/+15% Skill Damage then a Flex support spot. You can also substitute in more DPS adventurers by equipping a Skill Share like Cleo's for emergency heals in place of bringing a healer adventurer.
If the content you're playing has unblockable purple attacks; you might want to go with a more defensive build like a main DPS + Axe/Lance/Staff for Defense/HP/Healing.
With auto teams, the goal is to survive and not timeout. They're generally setup with an Axe, Lance or both acting as the main DPS and providing team survival with their defense/HP co-abilities. Then there's a healer and a buffer enabling "Odd Sparrows" health regen to constantly heal since the AI is always getting hurt. The easier the content, the more flexible you can be with swapping out an Axe/Lance with a DPS.
As for stealing SP from your teammates, if you're killing mobs quick enough that your teammates aren't landing any hit then you probably don't need to worry about their skills. Some solo content I run, my teammates are there just for their co-abilities to buff my main adventurer so my main can steamroll through all the mobs/bosses.