the new stug E is basically another brummbar at this point, with minor stats difference.
The vanilla stug E had two issue, it's too cheap (200mp 75 fuel 8 pop is a steal) and the main gun stun is too powerful.
I really have to stress how cheap the vanilla stug E is. At 200 mp 75 fuel it's only 5 fuel more expensive than the vanilla t70. The t70 might be the better infantry killer, but the stug E is just a much better overall package.
I think your solution is over complicating the issue by changing it into a new units.
and the doctrine itself have issue in 2v2+ because of the lackluster Tiger and Assault grenadier.
Live version Stug-E is a glorified, long-range T-70.
Unlike T-70. Unlike the T-70 though, this unit lacks a turret and, thus, the ability to safely close the distance. Thus, if the map happened to have any sort of elevation between the Stug-E and the target, tough luck.
I have played a lot of 3v3+ games, all steps of the ladder. I have never, ever, ever, ever, ever seen that commander bring anything but misfortune to the guy that chooses it.
There's so many hills and shotblockers and anything, that you are better off spamming Ostwinds (which arrive earlier than the Stug-E for those game-modes).
Even TWP is not enough to carry the Stug-E there.
Even if massed Stug-E can somehow displace Brummbars (which don't die to 3 shots poking them, btw), let it be. In any case the doctrine also gets access to Tigers.