Keep in mind that it costs 120 Munitions to get that upgrade. Increasing the fuel cost is pushing back the Soviet ability to reinforce on the field. Quite dangerous for those who use Conscripts and Penals (No call in infantry).
Of course, there's also another possibility we can pursue. What if we moved it's suppression buff to Vet 3 rather than have it from the start?
Soviets generally are not as ammo demanding as other factions (since cons dont have a weapon upgrade). and a sov player will have around 220 ammo by the 6:30 marker if they own half the map.
Alot of people skipped T3 totally before the last patch, so im not sure it would be a massive issue to have to pay an extra 15 fuel for the M5. But might make a player take more care of it than yolo it in with a quad cannon. Because 2-3 squad wipes is worth it ect.
I dont mid your last suggestion either, but its unlikely Ostheer will have a shrek upgrade anyway by 6:30 if hes upgraded LMG's. The "force retreat" factor will still be there :/