One of the biggest problems with Supplies is that it saves food, but costs food. Food is very valuable in the early game, so in practice, you need to make a LOT of Men-at-Arms to pay for it.
So my first suggestion: why not just change it from food and gold, to wood and gold? That means it becomes more like a farm than an investment, and it should. E easier to justify earlier on.
Now, the other issue is, what if you’ve already made some infantry? Supplies is exclusively a long term tech right now, and with how precise and efficient the early game has become, it’s really hard to justify a tech that only pays off much later and only for one thing. Especially since MAA are basically guaranteed to take damage, as their resistance to bonus damage is one of their most notable attributes.
Another aspect is, I think infantry are the unit which benefits the MOST from healing. Knights can lose 2/3rds of their hp and still get off 2 or 3 attacks. If Infantry lose 2/3rds of their hp, they can often die before doing anything but block other allied units from doing damage. But from a cost standpoint, healing them is not very efficient right now.
So what if if Supplies would give a bonus to the healing rate on garrisoned militia; something like +16? Make it a flat bonus, so they can heal in towers, too. That way, an early force can garrison and heal up while new units are being made. This restores their value, helps maintain a force of infantry, and helps to justify blacksmith techs, which in turn justifies further militia investment.