I don't think the AEC timing is a problem, but it was one of the units designed to be excessively good to compensate for the complete lack of snares. Now that RE have snares, its Tread Shot ability should be moved to vet 1 and maybe the ability's penetration should be brought down from 1000 to normal pen values (like Puma's Aimed Shot ability). Just like the 6 Pounder should now lose its 1.5 accuracy modifier vs Light Vehicles.
See Katktof's post regarding tread shot.
As for the 6pdr, I disagree. REs arrive later than Infantry Sections and they cannot be a mainline infantry replacement since they were tweaked.
If only Pioneers had snares in Ost, and you had to have T2 researched to build pioneers, and you didn't start with pioneers, I would say the PaK should come with a light vehicle accuracy bonus.
Unless the snare gets moved to tommies, your actual infantry squads, that accuracy bonus is the only thing that keeps the AEC from being picked literally every single game as UKF. It makes the AT gun a viable way to defend yourself.