I've done a bit of research on that so that I could start building a digital synthesizer with multiple ribbon controller inputs. I would personally suggest you use a board other than the Spresense, because my impression is that it was built for high level audio applications and not for anything that might require sample by sample manipulation at high speeds. It's possible that I simply overlooked features that are only available in the NuttX-based development (i.e. not Arduino IDE, which is very limited), but I think the board is far too new and lacks documentation to develop efficiently.
That being said, my first thought is to use a Teensy 3.6, which has 2 ADCs (13-bit) with 25 inputs, 2 DACs (12-bit), a real-time clock (which seems to be handy for precise timings), DMAs which work with these ADCs/DACs (which allows you to read and write without holding up the processors), and a beefy +2k page documentation PDF from NXP, along with lots of support on their forums and website.
https://www.pjrc.com/teensy/td_libs_Audio.html
For audio applications, I think the Teensy 3.6 is well worth the €30 or so that it costs (and cheaper than the Spresense). If you want to go even cheaper, you could also try the ESP32 in combination with appropriate ADC/DAC boards and a multiplexer, which would probably run you on the order of €10 or so. Of course, both of these options don't include an amplifier, but that depends on what you want to connect to your converter.
If you're interested, we can stay in touch to chat about yours/my work.
