Give them 5 Stg upgrade once you unluck tier3 (with p4). Now they scale well if you upgrade them.
Having good scaling unit that outscales units that pay for their scalability is stupid, so I think this is good idea
It shouldn't be impossible to make it work. With veterancy, the "resource" is the time it takes to get that vet, and so, it shouldn't be overpowered for AGs to compete against upgraded infantry with only vet if the time it takes them to get that vet is long enough to justify it.
TL;DR Increase XP requirements to nerf AGs in compensation for their vet alone competing with upgraded units if it turns out to be a problem.
I'd personally doubt it will be though, all CQC units have some trouble once upgraded weapon proliferation hits and take a bit more micro to be commanded right, just AGs are particularly not worthwhile to do so at that point and they even have Sprint to ease commanding them right!