I do not understand why you care about RAID? What is it that you think it offers you?
Also, I am surprised that you would consider an intel NUC spec similar to as suggested in the Roon KBs as expensive compared to a NAS (QNAP or Synology) with similar CPU/RAM/SSD capability. I would have expected an equivalent NAS to be easily double the cost. I dont recall prices changing much here in the UK since brexit (maybe I havnt looked hard enough), have they changed that drastically in France? I cant see why…
Personally, I dont really see the point any more of RAID at home. In the past, sure (performance was my main reason with striping across drives).
RAID is no substitute for proper backups.
Modern SSDs (at least those I use) are both very fast and seem thus far more reliable on average than equivalently respectable mechanical drives, so I don’t fear drive failures. Anyways rebuilding RAID after a failure can be easy (stuff in the replacement) or sometimes it can be a pain. One hopes for the former
As for recycling a NAS vs recycling a NUC - a NUC can run Linux or Windows either of which would be the likely platform for whatever other media system you may choose. There are plenty of reasons that a typical home/SOHO NAS can be very useful, but few of them have much to do with storage and are more to do with the always on appliance like nature of the device. There are linux distro specifically for building a home NAS-like device.
The advantage of QNAP and Synology devices to my mind are they they already have quite mature collection of suitably packaged and easy to setup software adapted/configured for them along with a very rich web based management UI that is orientated towards mostly hands off administration of an always on computing device to provide you with a bunch of services. I used to used my QNAP NAS for running all sort of services, but recently I dont really have so much need of them and could probably live without them. Roon meant none of the media services were needed any more. Security issues have meant I am less inclined to allow these things more then the minimum internet access especially access from the internet and most services I run at home these days are now running as docker images on a Raspberry PI instead of as native installs on my NAS.