Bernie, my iMac running Roon Core is a Late 2009 Core 2 Duo with a HDD and for my day-to-day activity, the performance is identical to running Roon Core on my 2018 MacBook Pro 6-core with SSD. I have had 50k tracks and it works flawlessly. I run Audeze DSP and it does that fine as well.
There are differences between the two devices in things like “audio analysis” time which is a lot faster on the MacBook Pro with it’s more newer 6 cores. Initial Roon boot time is faster on the newer machine and I’m less likely to run into troubles with tougher DSP on my MacBook Pro. However, if you’re not facing any issues currently with the performance of the iMac and are satisfied, I’d suggest sticking with the iMac for Roon Core and save the money. I am not noticing any differences in library search times or latency issues across the 4 Roon Core machines I have around the house with HDD or SSDs from the last decade. Of course, for non-Roon activities, the SSD is far superior to Fusion Drives or HDD. My next iMac or any computing devices will be pure SSD.
Yes, backups are very important to maintain and the NAS does do a good job in mirroring the information. However, if that’s the only place where your where music will be kept, I hope you know that this situation isn’t considered a backup. You’d still need something for a backup.
For backup drives, I have both portable and desktop WD USB Drives (My Passport and MyBook) that I’ll occasionally use to back everything. Once the information is backed up, I unplug them and keep them safe sitting in a drawer, isolated from vibrations or any electrical connection. I have 2 separate drives that are used for backup and are kept offline.
You may want to look into that to protect your music and time investment of going through 600 albums.
If you end up sticking with the iMac for Roon, you may even be able to save money on the NAS. Perhaps a simpler 2 bay model?