The NUCs are quiet and small. You will want a much more powerful CPU. One with a heftier fan and cooling system. One that doesn’t have total system TDP of 15-30w, but more like 125w just for the CPU. The Intel Core i9 9900K comes to mind. You will want fast RAM, and the fastest NVMe SSD for the Roon database.
1 million tracks is about 100k albums, and at even $12 an album, that’s over a million dollars of music! I think budget is not really a concern here. Good thing, since this will be a very expensive “computer”.
It will also be noisy and hot, but maybe it can be liquid cooled. If you are doing all this, you probably want to skip the NAS and go DAS.
I’m sure there is a dealer or integrator that is willing to build and maintain this for a fee. I’m sure I could find you someone if you really needed it… or you can go at it by yourself. Be warned though, this is a very different task from buying a NUC and installing some RAM and drives.
You’d also need to make sure your ethernet chipset was compatible with RoonOS, but that shouldn’t be a big deal unless you chose to get exotic.