Cascode MOSFETs for high voltage
I may misunderstand what you may be asking, but "I want to get C5/C6 from almost zero volts when the FETs are on back up to half of HV+." may indicate that you believe C5/C6 are discharged when the FETs, Q1 and Q2, are turned on.  This is not the case.  Also, just in case HV+, or in other words the bus voltage from the rectified AC lines, is not the highest voltage that Q1 and Q2 will experience.  Note that is a flyback design so the maximum voltage that Q1 and Q2 will experience is a little more than HV+ plus the voltage drop of D13.  I also second the response from Autistic, values are helpful particularly the MOSFETs you would like to use.