I came up with a new idea while thinking about what to do with the interference.
When I was playing with my oscilloscope and was running fast PWM by mistake in mode 7 (OCR2A holds TOP, OCR2B controls duty cycle) I noticed I can get much higher frequencies by dropping TOP (ofcourse!).
Now, 62.5 kHz is maximum output frequency with 'normal' fast PWM when prescaler set at 1. But my thinking is that the higher the PWM frequency is, the cleaner the signal. Setting TOP at 127 for example, you get a PWM signal of whopping 125 kHz ! And due to the way that the PWM mode operates, this means that output is maxed at 5 v when OCR2A=127 and OCR2B=127.
In summary, by using this trick (fast PWM mode 7) it should be possible to get a better PWM frequency for DDS synthesis while sacrificing DAC resolution (example 127 levels instead of 256 yields 125 kHz signal, 64 levels of DAC would give 250 kHz signal).