I think 16 pop is good since its an emplacement which naturally makes it much more durable than the pak 43. Making it lower than this could see it abused in large team games quite badly where some brit players will make the 3 three emplacements in combination to defend a point against all threats making it extremely tough to counter.
Try to gain building control in key areas like near fuel/muni points or cutoffs.
Since you can spam ostruppen out from the start send them straight to important areas of the map and use your pios to cap.
A build that may work is ost->ost->ost->mg->t2->scout car->scout car.
From then on its up to what you need, usually pgrens work pretty well with ostruppen and if you ever need more infantry firepower don't be afraid to make grens since ostruppen aren't as strong late game.
RE's are fine as a combat unit with weapon upgrades especially since they are designed for a support role not a combat one.
The main problem is the volley fire ability which penalises their received accuracy so much. It should be reduced so that RE's can be effective in a defensive role for warding off enemy troops while rifles maintain their aggressive role.
Yes! The Hotchkiss would be great idea for a tank in an Ostheer or OKW commander.
As for the panzer 3, a tank which was used so widely in the war should really be put into the Ostheer tech tree. If I had to choose between the two the panzer 3 should have priority imo, more tier units and less units locked behind pay walls.
They even pen the front of heavy tanks surprisingly often but only in close, which is why you have to do those ballsy all in dives with 3 or more m10s.
Armour is already a good commander choice but you have to be careful in large game modes like 4v4 since there will usually be more supporting AT when you try to flank or dive with m10s.
They are really unnecessary and over the top. Those 3 okw units are fine as they are, only real problem falls have is the high call in cost and bad squad spacing which leads to wipes.