Maybe not quite what you are thinking of using but I have been using Synology NAS devices for years to solve exactly the same problem. They do some very simple two slot devices but you can scale up to bigger arrays if and when you want. You can put a variety of drives in the box. They recommend you use identical drives but I have a mix of drives (some from old retired systems) and it all seems OK. I personally wouldn't use less than 4-bay units.
I started small with a four slot box and chucked in some old drives which it was happy to RAID up and vend on my network as a sharable file store. It does AFP and Samba. None of my Macs has trouble mounting them going back as far as OS 10.8 right up to the latest.
Config is via a web based admin console. That also supports downloadable native apps to add features such as mail serving, media managers and databases.
What I particularly like about them is the reliability. They alert when a drive is starting to go bad but has not yet failed. Replace it early and away you go. Best reliability is with two disk redundancy but you can configure for 1 drive failure to eke more space out of it with a risk if a second drive fails at the same time (I've never seen that happen but it could).
They can also provide Intranet and DNS services on your local network and lots of other good stuff if you want it but you can leave all that turned off if you don't. Basically they are a headless Linux PC with very good disk management and file sharing.
They take spinning platters or SSDs whichever you prefer.
I tried WD, dumped them and went to LaCie. Then moved to Drobo (very bad experiences). Finally went to Synology on the recommendation of Skott Kelby who made the transition after Drobo trashed his entire library of photos.
In 10 years or so, I've only had three drives go belly up. That's out of 40 drives spread across 5 Synology boxes. Replaced each of them with a new drive and the RAID rebuilt itself with no observable loss of performance within 24 hours.
Also migrated from a 4-bay unit to an 8-bay unit by simply moving the drives across and gradually added four more.
Now, I'm gradually replacing 4TB drives with 8TB drives and I expect these boxes to support 14TB and 16TB drives in due course which will massively increase the capacity in very gentle steps as the drives become affordable.
There's lots to like about Synology. There are other alternatives like QNap that are similar so I've heard but I don't see any advantage in replacing these boxes other than for scaling up.
Definitely worth a look. These days, they are shipping refurb units at lower prices and you'll also see them on eBay.
Cheers
Cliff