If you want to maintain a constant OA flow rate while varying total flow in a VAV system, you need to make 2 changes:

1. Specify the DesignSpecification:OutdoorAir object name in AirTerminal:SingleDuct:VAV:Reheat.
2. Make a Controller:MechanicalVentilation object, set Demand Controlled Ventilation to "No", VentilationRateProdecure system Outdoor Air Method, and reference your zone name and corresponding DSOA and DSZAD objects.

I don't believe you can make either of these changes using HVACTemplate; you probably need to edit the IDF directly.

With these changes made, you can set Constant Minimum Air Flow Fraction (in AirTerminal:SingleDuct:VAV:Reheat) and Minimum Outdoor Air Flow Rate (Controller:OutdoorAir) back to autosize (or not, it'll still maintain steady OA flow).