A mechanized vehicle (250 HT with flamer upgrade) for Battlegroup HQ would be very welcome if you want to get rid of the Luchs /Puma into Heavy call-in.
I can understand your opinion about the Sturmtiger but the unit is imo just not reliable thats why i would prefer the Brumm even above the much better reworked Sturmtiger version.
Part of the reason they removed Radio Relay from Strategic Reserves was the lack of voice lines for the ability so yeah...
I think the StuG III E will be your best bet for a new assault gun for OKW to be honest, again, just an opinion.