Relic 3 is bare minimum, r5 is what I consider the sweet spot, not too expensive and allows greater use from datacrons. The starting line up in any fleet deserves luxury Relic levels imo, beskar mando and cad bane as you mention are very smart investments. I personally got CLS to r7 as for a long time the stat sharing was invaluable when the rest of the team was a g11-12, now they are at least r5 it has become invaluable on offence for me in all game modes
For me, there are TWO factors on what I consider a Relic'd enough level: 1. If all characters meet the requirements to get the Deluxe character/ship I am chasing after. 2. If a team works for what I am using them for...usually R3 or R5. Loved teams (Darth Revan and co.) R7 minimum!!!