Have you opened up a Control:10/15 yet? There isn't much room in there. It's been a long time since I was inside mine, but the mainboard is attached to the back of the screen. How do you plan on mounting a Mac Mini, are you just going to use the board? Still may be a tight fit.

What touch monitor did you order? Does it work with Mac? Will you need a separate power supply from the computer?

A NUC might be a better option as mentioned. They're physically smaller than a Mac Mini so may be easier to fit inside. They can be ordered with Windows installed so running Roon client shouldn't be an issue. I'm sure the touchscreen would have Windows drivers.

The optical drive is obviously in the bottom separately, not sure if you want to use that portion?

Are you using this as an audio endpoint or just for control?