Reloaded to final Belgorod turn for a different approach to reinforcements and got a Pz IIIM for my troubles. After some more selective reinforcements (mainly using normal reinforcements if the unit would still stay with its current number of stars and only then use elite reinforcements to overstrength them) I ended up with 0.6k prestige this time. Just enough to upgrade another BF 109G to a Fw 190A, leaving me with exactly 10 prestige.
Went on the offense and send 4 units south to help the Italian in addition to 6 units clearing the partisans in my back.
It might just be my imagination, but the "chess" results look a lot better to me. And it looks like I found where the Iron Crosses grow (well, at least EK2s)

Took Medvezkha at turn 12 and left Pozdeevo open for a few more kills.
DV at 20/20 - 2k prestige - after the 7k scenario bonus and elite reinforcements (normal for units which would keep their stars that way, only then overstrengthening) prestige is down to 6.3k. Will sell my SE Grenadiers (too slow for my taste and can hope for another tank this way) during the next setup, so prestige before upgrades should be a bit higher. Not sure how I should feel about basically losing 0.7k prestige during the scenario itself, but it is ok I think, so the overall difficulty looks fine.
Gained 3 heroes this time, the heroless units with the highest number of kills has 172, as mentioned before IMO there should be a guaranteed hero earlier than that.