The TXCO's are impressive, but not really needed. There are quite some common xtal oscillators with jitter of around 1ps or better. Jitter becomes relevant for the ADC noise when the jitter is higher than some 5 ps. Old style 74HC... have jitter around 2 ps and the difference between a 74HC74 and 74AC74 is barely visible. Similar 74HC4053 gives slightly more noise then 74LV4053, likely from more jitter.
The µC internal PLL may well be the weak point. I have not found jitter data in the RP2040 datasheet. However they mention the compromise between jitter and power consumption for the choice of the VCO frequency. So jitter likely is somewhat noticable. The DS for the STM32G071 (a possible alternative µC ) gives jitter specs of typical 40 ps RMS period jitter. This is quite a lot and would be a real issue. That µC would need to run without PLL and / or use external synchronization. The RP2040 may be slightly better, as it is not so much trimmed for low power. However I don't expect is to be more than 40 times better than the STM32G to reach the 1 ps range. For the ADC the frequency offset is in the 100 kHz range and thus a more difficult case than period jitter.
There are also different forms of jitter spec, this from a Epson part, SG-210SCBA (0.3 to 19ps depending on jitter type)
plus you also need to worry about possible micro-step effects of crystals.
PCB.Wiz: Once again, thanks for providing more information regarding low-jitter clock sources. This is something I will be looking at in the future.
It's always interesting to do a refresh.
I found quite a few parts have 'come and gone', and it seems you can get high spec 2520 package parts for not much premium.
The appeal there is you can easily try a high spec part and see if there is really any measurable difference.
It's even possible to have dual operation, with High-Spec faster source user switched with a 12MHz Xtal or osc for booting.
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