In regards to class balance I always see it like this:
You need to balance all the classes within a role, then you need to balance all the roles that compete with each other (Namely DPS.)
When it comes to offensive balance, the only numbers that need tweaked are the RDPS + PDPS of one member of that role with the RDPS + PDPS of the other members of that role. An okay Example being DNC against MCH (although I would argue that MCH should be a little higher and BRD should be closer to DNC.) The DNC's Raid damage makes up for the MCH's Personal damage.
Where SE starts to go astray in my eyes is why they try to balance Offense + Defense vs Offense + Defense. Classes don't need to completely homogenize, but if a defining feature emerges from the role, then the entire role needs an equivalent. An example being again Shield Shamba and Tactician. They bring that same core universal Defensive buff with a long duration on a 3 minute cool down.
For casters what this means is giving BLM a raise and to do that they need to change it to compensate for infinite BLM mana by removing its MP costs and putting it on the OGCD. This would also be a QoL buff for SMN/RDM, who will no longer lose their Swiftcast or Resource Generation. Limiting it to a 2 minute cool down and adding 2 charges on it would allow casters to support without being broken. Any further support brought by a member needs to be good enough to use, but not so good that it makes the other classes undesirable. Curing Waltz vs No secondary defensive utility on MCH.
"All the casters having raise would make them mandatory!" Well geez I hope so! My second point for balance is that all roles need to have a balance with their competing roles. So all roles should be within a reasonable % of each other at the highest level of play. The party Role bonus needs to be what keeps everyone in a standard comp with a complete 5% bonus. You should want to bring one of each role for this bonus.
TLDR:
SE needs to change the way they balance roles. Core Non-offensive utility needs to be shared amongst the role. PDPS and RDPS should be the only factors in balancing Offense within a role. The party bonus is what should keep all the roles desireable in raids even if their DPS alone are similar, the diverse role choice boosting the party's DPS with its bonus.