[DoctorOetker]

Why not use the ADC's used for CIS (contact image sensors, the linear sensors in consumer-grade scanners / photocopiers)?

For example:

12 MHz (i.e. 12 000 kHz) sample rate per channel, 4 channels, 16 bit ADC:

https://www.akm.com/eu/en/products/mfp-lbp/ak8471vn/

single digit dollar unit prices:

https://octopart.com/search?q=AK8471VN&currency=USD&specs=0