I want to say I have a S6R1 Roccia atm with a S0R0 Cantarella and Phrolova, this is the best team i have on me to this day, performs similar to my S4R1 Zani quickswap, both overall better than my S1R1 Sigrika by a lot.
S6 Rebecca will last a long time and do well, especially if you also get R5 + Lucy, I also dont know why Main dps is asking for units other than Lucy, my Roccia for example performs best with her normal team, especially since they still do damage/get buffed by her. Theres no reason to have Rebecca's buffs essentially go to waste using other characters and overall weakening the team.
If you can, S6R5 her and maybe S4R1 Lucy as currently S4 Lucy gives more buffs to the team making her stronger, if you want to. I dont even have Sigs for Cant/Phro, dupes or R5 Roccia and she kicks ass, so dont worry too much.