So the issue here is that Crom and Kasima are significantly better than Antonius.
Crom provides 18% Atk and 23% HP alongside a 10% construction bonus.
Kasima provides 16% Atk and 20% HP, alongside 10% HP and ATK in two flaks of your choice.
Antonius provides 3.5% ATK and 10% HP to all towers. As well as 7% ATK and 10% HP to your choice of Earth Flak or Storm.
This makes him strictly worse than Crom or Kasima. If you wanted earth flak boosts you would get Kasima. If you wanted to general boosts you would get Crom.
Additionally I am not convinced that boosting the earth flak is the way to go anyways. While you do want the earth flak to tank as much damage as possible, its often one of the first targets for death gaze style spells, which renders any extra HP you put into it kind of moot.
If you wanted to make Antonius better I would think the way to go would be to slightly boost the first tower attack node to be something more like 7-10% Attack (most players go for HP over attack with defensive riders, its usually better to have a tower survive an extra hit and get a second attack off, then deal a slight increase in damage). Then massively increase the total boost you can get for your earth flak to something like +15% earth/Storm Atk, +20 or 25 Earth Flak HP.
That makes this rider a more interesting choice. You can significantly boost one important tower on your base, that you want to survive for a long time in favor of reducing the power of the rest of your towers. I don’t think I personally would go for this rider, but at least there would be a use case and a reason to go for it, whereas right now the rider is just strictly worse than existing defensive riders.