The interesting thing is that if a player tries to upgrade and distribute "new" AT early in the game, you can counter it by bringing artillery or just more infantry to overwhelm him. Without deeper espionage mechanics it's going to be relatively easy to conceal production and division design, even to the point you can stockpile equipment and only deploy it when there's a need/when you have army experience to spare.
Indeed, and from a game theory perspective, the loss is greater for the person making the expensive tanks than the person filling its divisions with cheap AT. If the meta becomes adding AT to every division, that will cause armor to be ignored.
Multiplayer games might turn into infantry spamfests.
And since the impact of producing AT brigades isn't too great, given its cost, they could still recover if an opponent does not make armor as you state.
That is why I propose giving HA an advantage, just for one year, so as to promote its use even if the meta is for everyone to include AT brigades.
Its high cost, for HA, will be the balancing factor. No one will be able to both spam HA and maintain a large enough force to cover all flanks.