Being cheaper isn't going to make the AEC more useful. The british have the weakest anti-tank capabiliy in the early-mid game with their lack of easily accessed tank snare and the lack of tracking projectile inf ATW. Any axis player should know how easily an atg get flanked even with faust.
furthermore, being useful only against light vehicle is a very niche and limited role. The AEC needs to make puma level decent against tank to be worth the purchase. Raise the price if need be, but the AEC needs to have 50 range at the least. It is the same exact problem the PTRS have. Light vehicle is a very small subset of unit. A unit that's only decent against light vehicle is just not worth using.
Schrecks are an easily kited hard counter, schrecks do not immediately invalidate all vehicles on the field. This is what micro is for.
schreck can be kited if the vehicle in question is decent against infantry (sherman), or if the main gun have good range(jackson, puma). The AEC have neither the range or AOE. The AEC can technically kill infantry but it's far below the capability of the t70 or even the stuart.
The remote chance of sniping an infantry model is not worth the risk of using AEC against schreck.
222 can kill nothing but UCs or halftracks vehicle wise, and is pretty bad vs. infantry as well (especially because everything more than a single squad will straight up kill it if you dont pull back. the only upside the 222 has over getting another gren/pgren is that it's not bleeding you MP as long as its alive (and it won't be for long, because it has no "window" unlike the AEC). 251 is bad because its a mobile reinforcement bunker without the flame upgrade, and a 120 muni sink with it, as it will go down to small arms as well (also no window). the russian clown cars are better because you don't have to upgrade the halftrack, but just put a flamer inside that you can save even if the vehicle dies. also quad.
I will agree that the 222 is a bit crappy beside hunting m20, and the flamethrower upgrade on the 251 is severely overpriced. the 222 could use a small armor buff and an acceleration buff.
the flamethrower upgrade on the 251 should be 90 munition