What if they just made a side grade that both flavours of grens got +7 HP per tier unlocked?
Explosive weapons destroy all squads with RNG but it would make then slightly more resilient and cost efficient without making them dramatically different. I don't really like the idea of giving a unit more than the traditional 80hpts but it seems the least invasive way to give them a slight buff that doesn't throw things out of whack
Your standard bolt-action rifle in CoH2 does 16 damage, requiring five shots to kill an infantry model. A +7 health increase means that those same rifles now require six shots to kill, making your infantry 20% more durable against all similar weapons (a nerf to Conscripts, Infantry Sections, Grenadiers, Obersoldaten, etc.). A second health bonus of the same amount would bring durability against non-bolt-action rifles up to the same level.
By somewhat solving the squad-wipe problem for two units, you've made those units more durable than they need to be, throwing off infantry balance completely.
A simpler solution would be to just modify all artillery weapons and tank guns to have a small or non-existent kill radius against infantry while being able to hit them more consistently or move most vehicles' anti-infantry capabilities to their MGs. This would avoid throwing off infantry v. infantry or vehicle v. vehicle balance and would probably also improve the issues with light vehicle balance.
Improving the squad spacing of four-man squads would also help.