One of the considerations I mentioned was that as well as Roon I need to run Plex for my Videos. Unless I am mistaken, ROCK won’t allow this.There are a couple of considerations mentioned by the OP that make this a relatively simple choice. Firstly a NUC running ROCK is the natural home of Roon. It is the simplest and least vulnerable solution. Minimum i3, 8GB ram, OS on a small M2 and music on a 2TB SSD and you have an excellent platform supported fully by Roon. The second point is that unless you get spendy a NAS will seldom meet the minimum requirement of Roon. This is a much less important point but a pretty interesting fact about Qnaps recent Roon related woes was that Roon basically shrugged their shoulders once they recognised the issues were not theirs to fix. The question is however, which NUC do you choose? I would say hunt down a gen 7 or 8 model. These run the same motherboards as the two versions of the Nucleus models. You can expect ROCK to be unconditionally stable on them. I run a 7i7 and I must be close to 100 days since my last hard reboot.
Seems to be about 50/50 as to whether I stay with the NAS or go for a NUC.