Thanks. The test circuit I was running was just wiring Vcc and GND into alterate ADC channels without any resistance. So I can't see how a buffer would help. Of course in the real circuit I will need buffers anyway (individually or as part of a clip detect circuit) so the ADC does not taint the audio signal path.
Maybe I should surrender and go for the 12xOpamp peak detect circuits, a 12 channel I2C DAC and sample at a much lower rate, allowing the peak detect to do the hard work.
I just thought that using 1xExternal DAC + WS2811s I would save on a lot of ICs and support passives for the project. This doesn't look possible though.
I could invest time in learning a more powerful STM or PIC uC with a much faster DAC, but I don't fancy that approach either.