An alternative way to fix that could be to cap how much damage one unit can take per one shot, so getting close with a shotgun could deal more damage, but you could still design it so they wouldn't die in a single weapon use.
I've planned to add more different resistances and such
Maybe you could have some other equipment which would modify weapons, adding perks and additional damage types to them (maybe some amulets, or something). It would give players more customization options instead of limiting use cases of every gun.
I am sure with enough time, and playtesting you are going to be able to balance things out.
