I personally just use an old desktop (4th gen i7, yeah that old) and a NAS (mostly for bulk storage seperated from the server). The device you listed is probably more than enough to handle everything, unless you’re sharing Nextcloud and/or Plex with a bunch of people.
Have you looked into an actual NAS rather than a mini-pc though? It’ll give you more storage upgradeability over a mini-pc, and a quality NAS could probably host everything.
I would also consider buying used, especially first starting out. You can save some money buying a model year or two older hardware for decent savings.