I have managed to replace both buck DC-DC converters with AMS1117 3.3 & lm317 linear regulators and I successfully got rid of the mentioned 1 MHz buck converter clock to obtain as pure a signal as possible. Do not expect miracles as the DAC's are 8-bit and have a best case SNR of -48dB, but getting the sideband modulation out of the signal surely improves the resulting spectrum a lot.
Well they might be 8 bits, but they're massively oversampled for HF, so with good filtering and noise shaping, you can get more SNR.
Cheers,
Sylvain