Wait, why are you running Dark team again? Go Fire. You literally need like one more Fire SSR for a near-perfect team. Apparently you're willing to cash so you can pick it up with a Miracle Ticket. Healing and debuffs are the core of any team. It's pretty easy to sub in whatever once you have those covered.
Otherwise, sub in Amaterasu. She'll benefit your team more than any dark SR while you're missing critical debuffs and/or healing. Then run Joan like I said above, or Mordred with ATK down.
Echidna LB1 is likely a small DPS boost. You can pair her with Dullahan friend eidolon for a nice HP boost without losing much damage.