Iirc I was starting to get some consistency with an ~R4 average on the team with Shaak + Rex being on the lower relics and Fives + ARC being a bit higher (Fives stat sharing is super important and you wnat to have at least 1 of the damage dealers quite high). This was ofc with knowing how to play and putting optimized mod setups on that team.
I wouldn't try to go for a bare minimum. You might win with a full R2-3 team if the stars align but what's the point if that's the case in like 2/10 battles?
Advised minimum:
Shaak - R3
Rex - R3
ARC - R5
Echo - R3
Fives - R5
For more consistency:
Shaak - R5
Rex - R3
ARC - R7
Echo - R5
Fives - R7
Easy life:
Shaak - R7
Rex - R3
ARC - R8
Echo - R7
Fives - R7-8