Another request for hardware advice/opinions.

Hi, I'm running out of room on my rock64 nextcloud/plexmedia/nas that i'm currently running, and considering building a dedicated NAS server. I abandoned/never finished a freeNAS build 7 years ago because I found out about the raspberry pi, and have since graduated from the pi to my current rock64 which runs perfectly fine. Except that my original 1TB usb drive is nearly full after all these years(even though i've been deleting items as deemed no longer needed). The company I work for has slowly switched from actual computers to the Dell version of the NUC as they die, and have slowly accumulated a pile of E-waste that is fair game. I have 20gb of ram , 4 1TB seagate drives, and the choice of a Dell T3500 or a Dell T5600, both with Xeon CPUs. I also have a Dell PERC H310 8port 6Gb/SAS Adapter, so to finally get to my question which MB would you suggest I use. After piddling with both i'm leaning towards the T3500, but the T5600 is a dual CPU board. My plans for this NAS is basic home use, pictures, home videos, files, and backups of computers and cell phones. As for now the only plug in i'm considering is Plex media server to serve the pictures and home videos. I'm currently planning on leaving my rock64 as my nextcloud server and just backing it up to the NAS once it's built. Thanks in advance for any input.