I'm about to give up on my old Synology 411j NAS. It works, but it is sloooow. It has 4 WD Red 4TB disks. All still fresh.
I mainly use the Synology NAS for a NFS share with autofs and fscache. Works, but it is way too slow. And only 1,5TB free.
I have considered for quite a while what to replace it with. Finally I have decided: Odroid HC2 and a single Seagate Ironwolf 12TB disk.
The HC2 is on its way and I have the Seagate Ironwolf on my desk.
Short term I will put the WD RED 4TB disks in an old PC enclosure and use them, and some other disks, as a backup storage server for my new NAS.
Long term I may use the surplus 4TB disks in a beefy combined home lab server / NAS that I can use to experiment with VMs and Docker and so on. And use the Odroid HC2 OMV NAS for backup storage instead. It is even portable...