Further responses...
In the case of Find Greater Steed, the player wants to use it to summon a creature that the character can direct at will, that also will serve as a mount. That doesn't seem like it's on par with the other two.
I just double-checked those two spells, and they both give the PC an ally that will attack targets it considers to be hostile to its friend. Neither one allows the PC to dictate the hound or charmed monster's actions. The monster also gets a saving throw in the case of the "charm monster" spell.It’ a 4th level spell. It’s in the same class as Mordenkeinan’s Faithful Hound and Charm Monster, both of which make a minion that can attack at will.
In the case of Find Greater Steed, the player wants to use it to summon a creature that the character can direct at will, that also will serve as a mount. That doesn't seem like it's on par with the other two.
Using a spell the way it was intended to be used isn't nerfing anyone. If it really is the intent of the spell to give the PC a mount that they can command and still do all their regular attacks, then fine.If you don’t like how they use it, don’t nerf them, give them a challenge meant to counter what they are doing.
It did take up a spell slot in this case, because the party were in rowboats far from shore, and the PC had to use a slot to summon the steed.However, don't forget this is a cast and forget spell. it's like Find Familiar, it has no time limit. Once cast, the steed is there until dismissed or it dies. It doesn't actually take up a spell slot.