Kafka-Sampo makes for a decent DoT core, it's improved by Black Swan if you want her. You should also get Ruan Mei or Robin, who do wonders for DoT and are the 2 best characters in the game. Robin also improves follow up and is a boon to Ratio, but you want someone who also consistently debuff to use him.
Ruan Mei is the best break support, you already have HMC levelled up, HMC/Ruan Mei/Xueyi is pretty good for AS, since Xueyi is a break monster, and a good introduction to the break playstyle to see if you want to invest in it. The healer here would be Gallagher, who is a free unit iirc and would be enough sustain for a break team.
Always prioritize supports, a E0 support will give you more damage than a E1 or S1.
But you kinda want another limited sustain, Aventurine is the best sustain right now, for someone without a second one he's worth the pulls.
Herta is also excellent for Pure Fiction, and worth raising.
And remember that the difference between a team that can get 10 stars and a team that can get 12 stars is less than 4% as far as jades go, so getting 2 teams up to that point is better than overinvesting in a single team.