Depending on features:

Starters: Fractal define - mid tower- 8 official 3.5" bays. With plenty of open space for more.

Jonsbo cases are the most NAS-like.

OS: Easy button: FreeNAS. Maybe the newer TrueNAS Core rework. As long as you don't need the latest and greatest in features, and at this point probably a bunch of unfixed security.

Otherwise it's Truenas Scale- just avoid the docker/VM system. Its a complete cluster.

I dearly wish Cockpit Project was up to par for this.