if you reduce their vet requirement, that is not going to change them from their current state. they already vet quite easily thanks to their HP pool and good overall dps.
Each unit should value their price at vet0, remember Pfus? we went from expensive low efficient vet0 unit with incredible veterancy to priced correctly for vet0 unit with incredible veterancy.
Personally I don't like heavies, I'll not care at all if those were overnerfed and leaving the meta.
Heavy tanks are quite different than fussies though. They are insanely forgiving and thus much easier to vet. Repair times are the lowest they have been so even if they take damage they can be back on the front quickly.
Heavy tanks, being limited to 1 are more like a "hero" unit. They should be nurtured and grown across the late game and losing a vetted one should really hurt (more than just the resources)
They need to be viable and it's a fine line between being op and being not worth their cost, one that has been flipped back and forth over for years. We have had massive pop caps and massive prices and massive performance and the inverse of them all as well (relative to the other factors of course) and just tweaking those is difficult to produce good results.
We could try some lesser used mechanics to try and tweak performance, things like damage modifiers for example (increase health but also make em take more damage so they take longer to repair) to make em justified in their power. We could try economical tweaks, like a small resource deficit like the tiger ace had but much more minor to simulate to resource drain such a monster would be. We could try adding more utility instead of more combat power to bring it more in line with being part of an army
there are more options than just the running the same stats through a randomizer over and over and over again