I know its in a different price bracket but have you looked at these before: https://bela.io/products
I would advise against getting this board for DSP stuff.
There is no low level sdk provided for audio but rather an overly complex higher level audio system that is very badly documented and basically seems to be designed around playing/recording audio files.
The audio output is digital (pulse density modulation) rather than analog so it is hard to see what is going on with an oscilloscope.
The extra m4 cores are crippled because Sony have not implemented Nuttx SMP but rather just treat the cores as coprocessors that have no access to the kernel/hardware.
I got this board for DSP stuff and after about 30 hours of struggling with it have decided it is not fit for purpose.
Maybe they will sort out the software level in the future, the processor itself seems interesting.