Osttruppen is only available in one doctrine, does it make sense buffing them into Knight Cross level?
No, but ostruppen fulfill a role within the context of the commander they are in. Comparing ostruppen to conscripts, for example, isn't really worthwhile, because one is a baseline unit and the other fills a very niche role in a single commander.
The PPSH exists within the context of only a few commanders, none of which have any late game call in tanks. Its not fair to compare it to something like the LMG42, which is available regardless of commander. The PPSH offers a strong mid game presence for conscripts that helps them scale into late game, at the cost of heavy call-ins. IMO unique commander abilities deserve to have some added strength because they occupy a slot on your commander load out, which should feel significant and game changing. We're not talking about baseline upgrades, we're talking about game defining abilities only available via commander choice, and for that reason I don't feel like its a valid comparison.