I suggest that if classes are ever to be upgraded the upgrades unlock via a kingdom specific quest line that introduces that kingdoms mythic. It would just really hype the mythic troops and make the game world feel more immersive.
For the class upgrades I think simple buffs to the preexisting tree would work, instead of anything past 100. Like, counterattack gives you 4 on damage, or chaos storm summons an empowered storm where only the storms colour falls.
I admittedly have no clue if this is significantly more difficult than overwriting a variable with a new one after some other variable change (class exp), or adding a ‘hidden’ past 100 tree that changes the wording of the initial tree. It would honestly probably be easier to convert class exp to x5 hero exp, and then classes and using your hero still remain useful.
Thanks for reading, and for creating such a fun game.