When you halve it with a flip flop, you’re fixed at 50% duty cycle (regardless of the duty cycle of the PWM output of the imp).
The SPI part, from what I could see, doesn’t allow you to set the frequency (not obviously anyway).
The I2C one I suggested will absolutely give you 12 bit resolution in the 600-800Hz range (prescaler of 10 = 25MHz/4096/10=610Hz… 9 gives you 678Hz, 8 gives you 762Hz).
The accuracy of the internal oscillator isn’t specced so maybe you want to pick the middle one, or if you need something really exact, you could connect an imp PWM to the PCA9685 EXTCLK input and then use a low divider - that way you get a crystal-derived PWM base frequency. You’d need to play around with available imp005 PWM rates (set 50% duty cycle) and 9685 prescalers to find the best fit for your application.