Yesssss, making some good progress here! It's been a wild adventure already. I am very grateful Patrick already did the heavy lifting on the Audio Injector Octo card in his vGuitar project, because it took me a while to get it to work properly.
When I started to use some of his code I knew zero about i2s or DMA. Niks, nada, noppes. And to be honest, I still don't fully understand it, but after staring at the code for hours, I now have at least superficial understanding of how it works, and can go on to the next steps to be taken. Right now only 2 output channels are working using a modified version of circle's built in i2s output code. Patrick actually rewrote it into something he could use with the Teensy audio library. There is a chance I might make something similar myself in the future when I want to get audio input working. The reason why it doesn't work out of the box with circle's code is because the Audio Injector should be the i2s master, and the circle code is made for the Pi to be the i2s master. It took some poking around to get things working. But like I said, thanks to Patrick for figuring out the CS42448 initialisation part, and the part to make the pi accept a master clock from the i2s bus.
The second reason is that the Octo actually uses something called TDM, that is to my understanding taking abuse of the i2s protocol to transfer more than 2 channels. Apparantly i2s was only designed for transporting two channels simultaneously. TDM is a way to get multiple channels working over the i2s bus.
The entire project now seems very promising since it left the theoretical concept stage. I've been giving the gui some thought as well, and think I'm going to create it with two types of input in mind: the touchscreen, and a rotary encoder (and other buttons). You should be able to do everything with the touchscreen, but also be able to connect a rotary encoder like on the mpc1000 and s6000 to change values fast.
I'm working as a freelance video editor for 6 days a week in August, so development for this project might be a bit slowed down during that time because I will be mentally drained when I get home in the evening, and imagine I'll need my Sunday to recuperate from screen fatigue.