bp3/t4 unlocked munitions scaling for the stug would be right proper
I'd use it as sort of rule of a thumb generally in similar situations. Make those less expensive units pay for having some timed abilities that make them scale better to late game. Like either build higher tier, field something more powerful and shoot with powerful stuff for free, or keep paying for some extra abilities with lower tier units (it has to be correctly balanced of course). Those lower tier units are still so much squishier have less healt and so on that it won't sttop people from building overall better units, but will give some sense to those units already on the field. It will also require some micro to activate those abilities.
Of coure extra range with vet is another path that could just complement it.