I don't believe there is a right and wrong side of balance. There is a strong and weak, but what is right is what you as the player want to play.
Thus the quotes around wrong. It's not wrong to play what you want, but if your character under-contributes because it is mechanically inferior, it's bad for the whole party, and even for the campaign making it harder on the DM. It's not wrong, but because the game is imbalanced, it's 'punished.' (There are those quotes again: as in, it has negative consequences. Not as in it's meant to correct an unacceptable behavior.)
Being weak is not the same as being a trap, nor is it the same as being wrong. I am playing a Monk right now, I was not trapped into doing that and I am not wrong for doing that.
There can certainly be instances where a player wants an inferior or under-contributing character, but a balanced game would allow that to be done advisedly, and without dictating that only certain general concepts could be that way D&D-like games already have level as well as class, so, even in the nearly unprecedented case of balanced classes, a player intentionally wishing for an inferior character in a group and with a DM both willing to work with that, could simply play a lower level PC than everyone else.
If you consider the best subclasses of each class (probably Rune Knight or Eldritch Knight on a Fighter), the amount of imbalance goes down tremendously becuase that is a big buff for classes like Fighters and Clerics, but a smaller Buff for other classes. Further using an optimal sub-class for some classes like Cleric (Twilight) or Monk (Mercy) completely changes the dynamics of the class Balance.
That is increasing imbalance. What you are saying, and you're not wrong, is that sub-classes are imbalanced within each class.
So when we are doing this balancing act are we assuming optimal races too?
Each choice the game presents must be balanced with it's alternatives. Races should be balanced with other races, classes with other classes, sub-classes with other sub-classes w/in the same class.
This is important because the best races are a bigger boost for martials than they generally are for casters.
Imbalance tends to make imbalance worse. Class, especially as you level, is by far the most significant choice you make for your character. Classes are imbalanced. Sol, yes, an inferior class may well benefit 'more' from choice of race or feet or background - in a relative sense, since it has less going for it, any improvement is bigger proportionally.
So to achieve this perfect balance
Perfect balance is impossible. Less terrible balance is the only plausible goal without re-writing 5e from the ground up.
What levels should we strive for balance at?
One mistake classic D&D made was trying to balance classes across all levels.
Since all classes use the same exp progression, they should balance at
each level.
You can't do that as long as choice is at play.
Balance is all about choice. Improving balance means more real choices.
This argument that fighters are restricted to being mundane holds no water at all in modern RAW 5E considering all the supernatural options available to fighters.
It is a bizarre feature of 5e that no class doesn't cast spells, yes.
None the less, the issue of D&D making non-supernatural characters profoundly inferior remains, with the handful of non-supernatural sub-classes being inferior to their supernatural counterparts.