when it comes to vet5, two options if it stays in its format (straight up buffs):
have it hard to reach and units get ridiculous at vet5 - obers, CommandPanter, JP4....
or have it easily reachable but basically be a vet3+ - volks.
...
i think better solution would be make OKW vet 1-3 match all others and vet 4 and 5 give indirect bonuses like better sight range, new abilities etc etc only.
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IS2 is fine, brit emplacements and trenches should be looked at imo.
I'd be up for three easily achievable levels of vet and 2 difficult ones.
At the very least, the vet requirements should be lowered and the vet capabilities should be reduced.