This way I can use the expertise of Cygnal in the critical PCB layout around the MCU.
The post by J.Guy blows your bubble.

Can I now HOPE to get anywhere near 12 Bits ?
yes, it is possible
1) use an external ADC
2) use a multilayer board with the ground plane split into analog and digital ground with those connected only at one point and a reasonable distance between the analog and the digital section.
3) filter the analog signal to barely meet the frequency response required
4) decouple and decouple and decouple with low inductance capacitors
5) have your power clean
6) keep the temperature stable
7) hope and pray

Erik