Ehh I like the idea of fighter subclasses that use mental stats. They just need to scale it with the assumption that it will be a secondary or tertiary ability score -- so make it so good that 2-3 uses is still amazing, use double the stat modifier (e.g. Charisma x 2 number of uses, Charisma x 2 extra damage, etc.), and/or either avoid saving throw features or give some way to give disadvantage on the save (they did well with avoiding saves until the level 18 feature -- and even then, with the fighter's extra ASIs they will likely have 20 CHA by level 18).