I think it needs to be decided what the acceptable minimum suppression for an HMG is, then set all HMGs to at least that. I think that's about MG34 level.
Making HMGs different is all well and good, but the super low suppression HMGs like the Vickers and Maxim aren't fit for purpose. An HMG needs to suppress well. There are plenty of other attributes (cost, armour piercing, arc width, bonus damage, abilities) you can dial up and down to make them different to each other.
Then all faction need similar tools do deal with HMG and similar mainline infantry.
HMG42 is better than other HMG by design.
Maxim could use have the vet 1 ability move to vet 0 and make it improve with each vet level.