Totally understandable Patrick and I’m sure that you will be happy with your new purchase. In defense of the Mac mini I must point out that Apple’s new Mac mini has none of the limitations which you’ve outlined. No modern business/personal Mac nor Windows PC for that matter comes to my mind.
Practically any Mac/Win PC can serve as a server if it allows for network access depending upon your needs and of course you have to secure it. That’s the beauty of Roon. They really have put in a lot of work. Roon server will operate on most boxes. It’s nearly plug and play as I’ve mentioned. I know many experience difficulty in navigating the server and network aspects of the software but Roon’s implementation and creativity is impressive.
The Amphetamine app mentioned by @Jim_F isn’t required to keep a Mac from sleeping. Mac’s can do that on their own via the system settings. That wasn’t always the case (Jim’s just broadcasting his age and I mine). The app does offer some great additional features but nothing benefiting someone going headless.
In the end I believe you made the wisest decision or as close to it as possible, a Roon recommended NUC.