As the C19/R3 circuit will take 6000/2 = 2KA, that one big switch!. As the recycle time is 15 seconds, I would leave the charging resistor in circuit. Or if its power dissipation is a factor, power the 6KV PSU from a current regulated AC source. To get the 2µS rise time the switch/capacitor/R3 must be wired up to minimise series inductance, in fact I wonder if your capacitor by itself will be "fast" enough.
I would build the HV switch around the guts of a car starter solenoid, re-engineered to provide the HV insulation.
Frank