Proper balance would be every single job in the game making a perfectly equal contribution when played to 100% of its capability.
It's kinda impossible to hit though because some contributions can be difficult to quantify.
RDM and SMN both have raise for example. However in my opinion just having access to raise should not mean those jobs are notably weaker than BLM in damage.
In a fight where those raises aren't needed they contribute absolutely zero to the party, Where as in a fight where those raises are needed, they already come at the cost of reducing the damage those jobs contribute.
How easy or difficult a job is to play should have ZERO impact on proper balance. In a well-designed environment they might all have different skill floors or different skill curves but at the ceiling, they would almost all be perfectly equal in terms of overall contribution. Not really possible in this game for numerous reasons. *EPIC PAINT SKILLZ ALERT*

This would loosely represent proper balance.
Job A might be your average job with an almost linear skill curve. An almost direct correlation between effort and contribution
Job B might be seen as an easier job for beginners to pick up because it makes a greater contribution to the party for less effort. but at the same time after a point in player skill or effort, it becomes harder to optimize than Job A.
Job C might be seen as a harder job for newer or less experienced players as it requires significantly more effort to compete with jobs A and B but also ends up being easier to optimize.
But when played to 100% all 3 jobs make an almost equal contribution to the party. That's proper balance. So in theory given that MCH SMN and BLM all contribute absolutely nothing but DPS to the party. At the skill ceilieng they should all do the same amount of damage in order to make an equal contribution and thus be properly balanced, regardless of how easy or difficult they are to play.
Arbitarily making a job contribute less because its easier is flawed. people choose jobs based on thematics and aesthetics more than how easy or hard they are to play.
SMN is one of the most difficult jobs to play.... the difficulty isnt its rotation though its just staying awake...