Imo Mage is fine as it is, Defender is too, Commander seems fine too.
It's other classes traits especially warrior (and also assassin) that is a major problem. They take too long for them to be useful, granted you can make them an archmage if they have spellbook, but this doesn't serve their purpose as a non mage hero. Someone once said these classes traits are full of filler, especially warrior and assassin.
Warrior and Assassin can be good if you can reach the very end of their traits, but Mage or Defender or Commander can be good/useful before they even reach the very end of their traits.
From my experience, mage hero is fine, because you often just need 1 powerful mage, and the rest just mediocre as long as they each are an archmage in 1 school of magic. Actually i prefer having multiple mediocre mage hero rather than 1 powerful one. Same with defender or commander, they can be useful enough before they even have their late stage traits.
My suggestion is to swap some of the most useful Assassin and Warrior traits that lies at the end of their traits tree to make it available early, example is swap Warrior's Rage traits or Teleport attack or Decimate traits with traits like Lethal or other early stage traits. Or they might need some major rework.
Other suggestion is make hero level very fast, like 5 times the current one, but give a limit for max level. This way you can't have all the traits, so you can't make single or few super powerful hero, but you are forced to build each hero differently, making each hero unique and equally useful.
This also eliminates Warrior and Assassin hero problem (because if they level fast enough, their useful traits which lies at the end of the tree will be available as quickly as possible).
That being said, normal units should also have a limit for their max level. I think level 20 or 25 is reasonable. Most game like this usually impose max level limit, both pure 4x genre or pure TBS or hybrid like FE: LH. After all this is not a RPG game.
Or make leveling to level 10 extremely fast, like 5 or 8 times the speed of leveling from level 10 to 11 or above.
