Yeah SMGs are too good right now, they basically invalidate ARs and most DMRs. Meanwhile on the flip side you have LMGs which basically have no role, they're no better than a good AR in most cases with a few niche exceptions like the KTS100.
Frankly, LMG attachment cost should be reduced significantly that way there's a reason to use them: they're great no holds barred weapons platform that are a bit bulky and unwieldy compared to other weapons.
That gives them their identity.
SMGs just need their precision lowered, they're far too accurate at medium range to the point where their lower damage output doesn't balance them (considering how easy it is to get more ammo in BF6).