I know EXACTLY what you're looking for. (I was in this exact situation myself recently)
This is a SAS-based 4-bay QNAP JBOD for 3.5" SATA drives.
https://www.amazon.com/QNAP-TL-R400S-Rackmount-Enclosure-QXP-400eS-A1164/dp/B086WBHRCF
I personally wanted one myself, but compromised for something else, but this would be perfect for you if you're willing to spend the money.
Yeah, I use it regularly. For LDAP, you'll need to deploy their LDAP Authentication Proxy.
They have a nice guide written up on it on their site;
https://duo.com/docs/ldap
Read up, deploy in an LXC, and then point your LDAP traffic -> Auth Proxy -> LDAP Server.
That should do the trick.