AG has some issues to overcome in today's meta.
One of the issue, is that they lack good low level monsters to summon. Nevermind doing anything against meta decks, you're screwed since the start if you don't get something good you can summon early in the game, and even worse part, Knight, the only good level 4 or lower AG, is a Gemini meaning it takes 2 turns to be useful, assuming it even survived the first turn.
Even the instant Castle means nothing if you don't have anything to summon. It's a risky move waiting for the opponent to summon something so your Castle can gain a counter, while your monster zone is empty.
The only other level 4 or lower you most likely will use is Sergeant Electro, which not only is not an AG, it also has only 1600 ATK. It's risky summoning it on the 1st turn, especially since it has nothing to target with its effects.
AG not only needs AG Wyvern for low level option, but also, the archetype is better when played alongside Gadgets, especially Gold Gadget and Silver Gadget. Not only this allows a build focusing on AG Reactor Dragon and the AG Gadjiltron cards, but also, Gold & Silver Gadget are low level monsters that gather tributes into the field quickly.
As for supports, AG Catapult and AG Hunting Hound would be great.
Catapult is a spell that lets us summon AGG from the deck for the cost of just 1 any card, while Hunting Hound is Polymerization in the form of an AG monster, making it also a valid material for UAGG.
Ancient Gear Reborn would be great too, it's a Cont. Trap that allows the revival of an AG once per turn, with additional 200 ATK.
For fusion monster, the only one we need now is Chaos Ancient Gear Giant. If it hits the field it's practically over for the opponent, even for the 🔥 SSA deck.
Megaton Golem is awesome, but is too demanding to use as you need multiple AGG/AGG-UP for Megaton to reach its maximum potential, so that's why I think we don't really need it.
Last, since AG Hunting Hound already acts as Poly while on the field, we can replace the regular Poly with AG Fusion, which allows the use of materials directly from the deck if you control AGG. Very handy as it means you only need AGG on the field and AG Fusion in your hand, and bam, suddenly UAGG/Chaos Giant hits the field.
It's Level 4, so it is easily summoned, and while 1600 ATK isn't much, but being able to spit out AGG from hand ignoring Summoning Condition would be useful in situations where you have a dead AGG in hand with no other way to summon it.