MechaTarrasque
Hero
I just had a thought. If the problem is that we want the AC to keep attacking after one order, well, 5e has a mechanism for that: concentration. The animal companion will keep attacking a targeted enemy for as long as you maintain concentration (as with a spell) or until the enemy or animal companion reaches 0 hit points. Low level rangers will give up things like hunters mark to keep Fido fighting, and for high level rangers, it is probably worth it to lose an attack a round so you can concentrate, attack yourself, and have Fido attack.