If you are willing to build them all, with the toons you have, I'd try Stokk, Jarl, Geomancer, Nia & Rearguard. Hopefully you'll have enough sustainability to last longer than your current team.
Make sure to be selecting Warmaster/Giantslayer for T6 and LifeDrinker for T3.
In this setup, Artor would not be better than Rearguard due to wanting the Dec Def from Rearguard's A1.
Speed-tuning refers to having your toons act in the best order possible, such as getting debuffs up before a damage dealer hits, and in having them place their buffs and debuffs at the best times in relation to the Demon Lord's skills...such as Jarl placing Block Debuffs before the Demon Lord does his stun attack, and then (for example) having Aniri extend before it falls off of anyone.
Deadwood Jedi is the best resource for speed-tuning and his site provides a free speed tune calculator.
https://deadwoodjedi.com/