They should arrive with G43s, and then have their STGs as an upgrade (90-100 Muni).
We already had an unit like that, they are called Stormtroopers and they're doctrinal Infiltration units.However, they come out with 4 Kar98ks, not G43s.
In terms of Panzergrenadiers, they have been here since the launch of the game! They are Elite infantry, specialize in AI when unupgraded, AT when equipped with Schrecks... They are
Elite infantry, of course they can beat unupgraded Rifleman! Rifleman are mainline troops, multi-purpose when choosing upgrades, that's why they are versatile, but that doesn't mean they can fight toe-to-toe with
Elite Infantry without upgrades, that would be batsh*t OP! Not to mention the fact Panzergrenadiers are more expensive (340MP to Rifleman's 280MP), come out way later so that's why they are stronger.
The problem here is more due to the USF design. Their lack of non-doc Elite infantry is what hurting them. Have to heavily rely on Rifleman, so much that losing one mid-game onward would cause that destruction of your entire army, since USF relies heavily on keeping Rifleman alive (Every faction need to preserve their units, of course, but in the case of USF, keeping Rifleman alive is the matter of winning or losing the game). This problem happens to all WFA factions, which seriously one day, need a look at. In terms of the USF, maybe make give them an Elite infantry units non-doc (Rangers seem like the most suitable candidate, in my opinion), while nerfing Rifleman accordingly.