Setting aside the problem of how a 37mm gun firing an HE shell can punch through the armor of a PzKpfw IVH (with shürzen for additional side armor), there are other options to balance the FlaK vehicles against the rest of the battle.
The first option is to rip off the armor. The Germans made two FlaK conversions of their 8-ton halftrack, with an unarmored gun sitting on a platform deck behind the driver's compartment:
The SdKfz 7/1, with a 2cm FlaKvierling 38:
The SdKfz 7/2, with a 3.7cm FlaK 36 (this version has an armored crew compartment, but many were produced with the same unarmored frame as the SdKfz 7/1:
Implementing either of those vehicles, however, would require significant modelling work.
Or, rather than creating a completely new vehicle, bring out an earlier FlaK conversion of the PzKpfw IV:
The 2cm FlaKvierling 38 auf Fahrgestell Pz IV, Möbelwagen:
The 3.7cm FlaK 36 auf Fahrgestell Pz IV, Möbelwagen:
Both of these vehicles had to lower the side armor panels to create a fighting platform from which the gun could be fired. The 3.7cm version would require less modelling, as the existing 3.7cm manned AA position model could be used on the PzPkfw IV hull with some extension plates for the lowered superstructure sides. The exposed gun would allow the 'turret' to be killed as easily as the existing manned AA positions while the gun was in use (using the 'open doors' command to lower the superstructure sides, after which the gun could be used; with the sides up, the superstructure would be treated like an Ostwind turret for damage).
For a step between the existing Ostwind and the M16, there's the turreted quad-20mm version:
The 2cm FlaKvierling 38 auf Fahrgestell Pz IV, Wirbelwind:
If any of the less-protected FlaK platforms were implemented, I would suggest that the SdKfz 7 variants would be free, the Möbelwagen variants to have a perk cost around 5, and the Ostwind/Wirbelwind to have perk costs around 10. The perk values would have to be played with, but any non-zero perk value would prevent a FlaK vehicle from being casually driven to an enemy field as a 'disposable' vulching and field-destruction tool.
In the pictures above you can also see why the Ostwind is able to keep up continuous fire -- the table feed mechanism of the FlaK 36 allows additional five-round clips to be fed in as the rounds are fired, unlike the enclosed clips of the FlaKvierling 38. The US 40mm Bofors naval AA gun used a vertical slot ammunition feed, which allowed the same continuous fire.