In terms of big towers with fans, I’d guess that your Mac Pro is at the low end of scale.
This is what I do on mine (my work machine). It contains all the OS stuff and all my applications.
I don’t think there’s anything to be gained by using multiple SSDs. (Obviously, you want to be careful about backing up any drive that contains a lot of important stuff.)
I’m definitely not an expert on digital room correction — I mostly just read about it — but I have a couple of questions and comments for you:
Acourate is Windows only, right? So how do you see that working out in practice?
Do you need the Acourate (or any other) convolution engine running alongside Roon in real time? Don’t you want to use your DRC tool(s) to generate WAV files containing the convolution info that you can plug into Roon? (See, e.g., this very interesting thread.)