Yes - this is what a lot of us do, however to store DVDs you'll need to decide how you prefer to rip and encode them. To watch video content from your NAS, you'll need another solution. I use Plex which many streamers and smart TVs support. You can run the Plex server on your iMac as well and, as with Roon server, just point the library to the NAS storage for the content.
There are different routes you can take to enter the NAS world. It's possible to start small and learn as you go without spending too much money. Alternatively some folks prefer to do extensive research and much more on a solution that they intend to last them 5+ years.
One strong recommendation I would make is when buying drives, go for 4 terrabyte models. They currently sit in the sweet spot for $ per gigabyte and you will be able to simply add more 4 terabyte drives if you migrate to a larger NAS. Smaller drives may find you needing to upgrade either the drives or the NAS (to hold more drives) sooner than otherwise necessary.
Synology and Qnap are two NAS hardware makers that tend to be most often recommended. I have had excellent experience with Synology units (on my 4th) and dipped my toe briefly in the Qnap pool but my use for a second NAS disappeared which is why it's currently listed in the classifieds ... going cheap
