As has been pointed out, you're the GM so you get to say how this works. I'd probably just tweak the stats a big directly instead of trying to use items (because that approach lets you tweak the treasure independently of the stats, which is in and of itself important, just the raw materials in a full set of plate armor for a grown dragon is likely to be worth at least 10k gold even ignoring any value of the armor itself).
But AC is probably not the stat you should tweak here
AC effectively acts as a multiplier on the base HP values proportionate to how likely the attacker is to hit the target, because you need to make more attacks to deal the same amount of damage. The exact formula for this is
1 / chance to hit. Ignoring the possibility of advantage and disadvantage (which skew these numbers significantly), you get:
|Minimum Roll to Hit
|1 (always hits)
It should be immediately obvious from this that increasing an already high AC has a much bigger effect on how many hits it takes to kill the target than increasing a low AC (and, conversely, increasing the chance to hit a target has diminishing returns). This asymmetry exists in any system that uses AC rules equivalent to 5e (including anything derived from the 3.5e or 5e SRD), as well as systems that use % damage reduction instead of AC (which includes a very large number of video games).
The net effect of this is that you have much more precise control of how 'tough' your creature is by tweaking HP instead of AC (especially for creatures with both stats high or both stats low).
Of course, there's also another factor to consider, most players can only get up to a +14 modifier on an attack without magical assistance (+6 prof with weapon at level 20, +5 on the modifier, and a +3 magic weapon, barbarians can instead get +16 on melee at level 20 due to the STR boost from their capstone). This means, based on the above table, that against fully kitted top level players, an ancient red dragon still has more than a 1.5x multiplier on it's health due to it's 22 AC, and therefore an effective HP of more than 816 if you're using average HP. That's an insane amount of HP to have, even with optimal weapons and average damage that's still at least a 20 round combat to kill it with a party of four.
You're making a lot of assumptions here about how 'at risk' the dragon would be. Looking at your original scenario, only about 15% of the shots made by the elves would actually hit an ancient red dragon, and if 100 of them make a single shot each, that's an average of about 67-68 damage they would deal. That's a non-negligible chunk of damage, but the ancient red dragon has a d20 hit die, and it has 28 of them, which means on average this is a drop in the bucket for it (in most cases it will take it at most one day to recover).
Even if we assume max damage on each shot, a group of elves with regular longbows would actually need 460 elves taking a single shot each to kill an average ancient red dragon. In reality, you actually need more than five thousand to ensure a kill if you are actually rolling for HP for the dragon instead of using the average (max HP is 812, so you need 5414 such elves to ensure a kill if you can assume that at least 15% will hit).