Good point that its not limited to SMPS, thank you. EMI/RFI ingress can happen in various ways. Still, seems like using decent quality linear bench supplies as primary regulators and local-on-the-PCB linear secondary regulators is easier to get clean on first go-around than SMPS on the same PCB. That's all. In keeping with the incremental test idea, hopefully.
Mumetal box, then copper clad pcb box, the. Stainless steel box. Batteries inside for power.
That’s how Jim made his low noise amp and i suspect the same level of complexity to reduce environmental noise on the traces etc for -130dB or below..
For the front end op amp you may want to look at matched resistor network opamps (like the LT1991 or an equivilent).
I’m on the same journey but i have a prebuilt AKM 5572EN board. Not as low dB as yours but that’s bad enough.
The clock i was considering a thermal OCVX (if i have those letters in the right order), and thinking a super cap just for the clock power supply. So the cap is charged and the supply then covers the period of the sampling.
I was thinking a small linear 3042s for the PS with filtering before it. Possibly with a fast RF opamp shunt regulator.
However those are future ideas - i’m looking at usb-i2s at the moment using a baremetal rpi zero to act as large fifo buffer. Without the interrupt conflict of the rest if raspberry OS, it’s DMA should keep up easily.