My two cents to this would be:
Melee champion cavalry is over-performing, therefore it should either
A) be weaker (deal less damage, have less hp, get countered more by spearman)
B ) be more expensive (my favorite idea personally; put all champions of all civilisations in an extra building like the spartans/athenians/han already have, which discourages spam. One could also imagine just increasing the training time by 1.5x or the cost to 300f 100w 150m)
C) be countered together with all cavalry, by making defensive structures actually relevant (why cant I build 5 Towers next to each other? Seems like a silly restraint considering how expensive and underwhelming each tower is.)