If you want to control audio relays, look at a Teensy LC. IT's more than capable.
If you want to do any digital audio processing, even a "digital mixer" you need to consider a lot more.
Getting guitar signal into and out of a CPU is always much more involved than people think if you want decent sound quality, low noise (digital interference), proper impedance (guitars need very high impedance) and voltage protection (take the 7 or 8 Vpp signal out of an analog guitar pedal or active pickup and feed it into an ADC and you will fry it.)
You may want to take a look at the TGA Pro. I created it specifically so people who want to get into programming with digital guitar effects can skip worrying about all the preamp, impedances, analog supply generation, power supply filtering, voltage protection and get right to the fun stuff.
https://www.tindie.com/products/blackaddr/arduino-teensy-guitar-audio-shield/