Did you finally sucessfully measured your signal at high frequency?
I have a partially similar question about double ADC measurement here : UNO Q: Simultaneous ADC1 + ADC4 timer-triggered DMA capture at up to 2.5 MS/s
It may be possible to do that rebuilding the UNO Q loader, implemeting a function using there low level capabilities and exporting and calling it from an Arduino LLEXT sketch.