build my own board ?

Hello,

I want to build a (SMALL & NARROW FOOTPRINT) board that does audio DSP, so far, I tried :

-  using a teensy 3.6, but  it has a terrible ADC (50% SNR) and the I2S interface is glitchy at best

- I also tried on a RPI Zero, interfaced with an MPC3002, but, it's really hard to make the cpu access an ADC in real time (although going through many kernel and package optimizations)

then I wondered, is it that hard to make a board myself ?

- what CPU should I use (let's consider using JLPCB / PBWAY services to build the board) ?

- I would add fast/close to cpu ADC & DAC chips ?

- how would I implement the usb programming of it (or anything similar) ?

- is it possible to have it, arduino (or ATMEL studio) compatible ? you know, for c/c++ arm libraries (math, FFT...)

- should I instead use an FPGA ? I heard they were not optimized for DSP audio ?

just out of curiosity, I wanted to check that direction instead of ready made boards, that never do what I need

thanks for the time you'll spend on this question

Phil