A prerequisite for reproducing this project from scratch.   Probably will be an actual set of steps to build a box, but for now, I'm just playing around.

Otherwise, how could you have the time to do all this?

Since I live on a sailboat and have to transport everything by dinghy, small and robust is important to me.   The guitar rig this is intended to replace is an iPad running SampleTank, Quantiloop, and Tonestack within an Audiobus environtment, using an iRigHD usb audio device for guitar input and line out.  My main guitar is a Rainsong carbon fiber acousitic guitar with an added FTP Fishman Triple Play midii pickup that communicates with a USB dongle.   On the floor are 4 analog pedals connected to an analog-to-usb dohickey that is basically a teensy on the inside, along with a SoftstepII usb midi switch padal, and an akai MPD218 that I use as a foot switch array.    I also have one of the few ACPADs, that is like a midi-guitar-wing, that *could* provide a variety of midi controls over BTLE, but I don't think I'm gonna use it.   It looks cool though and has a bunch of leds, drum pads, and slider things that go on the face of the guitar.

The iPad rig has 38ms latency.   I'm not bringing a laptop on stage damnit, and I'm sick of big operating systems and their inherent complexity and delays.    No IOS.    No Wndows.   No Android.   Ok, a little android, but only native apps communicating to the bare metal system.   And no linux, though I could have probably saved 2 years of my life by just learning Jack and trying to minimize latency in a rpi raspian system, and would have gotten zynthian and a hundred other benefits from it, I, stupid as I am, have decided to do it all from the ground up.

Now, after you've done all that, give it 6 months ... just learn about the MPUs that are available, and many, many details about them and the periphals, and GPIO, and hundreds of protocols like I2S, I2C, SPI, and for good measure implement your own bluetooth stack from the ground up with HCI, L2CAP, SDP, RFCOMM, and SPP.   Learn how to do DMA and use a logic analyzer.  Try reverse engineering 2 or 3 poorly documented devices.   In general, mess around, A LOT.     It will be good practice at banging your head against walls so-as to build up a nice thick callus on that part of your forehead.

That's where I'm going with this ....