My advice is what I always give.
Do whatever you want, and don’t care about balance, things should be fun first, you can do that as you develop, making the early army mostly fliers just means you’ll need to design around it.
Another way to make a unit unique is stat caps in a personal class.
Ignoring gendered stat cap and base differences, which are insignificant anyway, you can cut gendered classes.
Since the only thing that determines gender is the Female flag, and it is only used in skillsys or for mounted aid calc, some classes that share animations like Knight and General can be made unisex or genderless as they do not need the mounted aid count, and even then great knight can be made unisex by either making mounted aid calc the same between gender or ticking the female flag in the character’s data screen as vanilla does it anyway…
Monsters can also be used if you do not use them in the game.
You can also remove one gender of a class if no characters use it.
You can also remove unneeded cutscene only and unused classes [like zephiel’s prince class from fe7]
FE8 also had like 3 copies of the trainee classes [one is regular, one is tier one, the last is super you can kill tier one and super, if you don’t want to use super trainee system and replace all outright if you don’t want to use the trainee system at all]
If you make gender nonconforming characters such as Forrest from fates or Rosado from engage then you can have them use the animations that match better, e.g. a feminine man using female sage animations, or a masculine woman using male hero animations.
Allowing you to cut even heavily gendered classes if needed.
I used these tricks to add 12 new classes 14 if you count blank space with currently up to 2 extra spaces available for classes while keeping all vanilla classes in FE6 which has much less class space and much fewer potential to cut classes.
Do keep in mind some classes like gorgon eggs and demon king have special hard coding so do not use those they will not work properly for a standard class, here is a thread that details the ones to not use.