I get the argument that OST has comfortable lesser Panzer IV (G) that is a safer bet.
It would probably be theoretically a better option to consider in the hands of OKW, but imo end up being neglected in favour of doctrinal Ostwind or Luchs simply due to its micro intensive requirements and underwhelming performances.
If an OKW player keeps a luchs alive for 5 minutes he gets a lategame killing machine that can suppress squads in green cover.
The Ostwind is a solid tank that can hold off t-70s and even Stuarts and AEC when properly supported
The Stug E is simply unrewarding, so even OKW, with its expensive premium Panzer IV (J) would have better Tier 3.5 vehicle options.
I think OKW would benefit more from the unit since the faction lacks a late game AI vehicle that can support in an ongoing fight. They have Obersoldaten for that, but sometimes you don't want additional infantry to reduce bleed. They go directly from Luchs to an expensive P4, so at least the StuG E would have a niche as an AI specialist above the Luchs if you don't want a P4 for the job. Doesn't necessarily make it a great unit, but the StuG E would at least fill a whole in the line up, whereas it competes with at least two stock units (plus alternative doctrinal units) in OST.
The extra range doesn't really play in its favour with the slow projectile and casamate set up. As a support gun, to my experience it's inferior to even autofire 120mm mortar and leIG options
I think it would probably work better as a direct fire turretless anti infantry tank with a su-76 barrage ability.
I agree to that, although I personally like the way units like the StuG E, Brumm and Scott function with their "manual" aiming. However I'd assume if it had direct fire it didn't need an additional barrage, but that would be up for testing.
Casamates may be less used in 1vs1, but they are definitely still present, especially Stug G, Jagdpanzer IV ( and SU 85 occasionally.
The reason they are less used imo is that heavy tanks are much less common as well, and since most (all but the stug E and Brummbar) casamates are tank destroyers this makes premium mediums like KV1, T-34/85 and Panzer IV, generalists capable of engaging all targets especially with atg support, a better investment.
I don't think it has to do with casamates in general.
These units are used less because in 1v1s they are prone to being flanked. Larger modes become more "laney", making flanking harder. In essence casemates keep their strength while their disadvantage to being flanked diminishes. In 1v1 it is basically the opposite. Obviously these are also map dependent, but the general point holds still true.