Using ReadyNAS for rack mount storage for backup, comparing to Synology

Presumably, you are aware of the noise level and frequency range of 1U rack-mount computer equipment and have an appropriate location.

You are really making poor use of a NAS if each drive is a separate volume and it's only used as "a stack of drives".  So having more capability on a higher end NAS (Netgear or other brand) is certainly a waste.  A SAN (which the ReadyNAS can also act as) would seem a better fit, but you're probably not going to find one any cheaper than the RR2304.  In that configuration, you will have to manually distribute the files across the volumes (unless Retrospect can do that for you) and, of course, you will have no redundancy.  Redundancy is less important if all the data is just backups (as opposed to archives) or there is yet another backup.  Still, it gives you the ability to work through a drive failure, which can be critical in a production environment.  But, you know what you want.

I did not mention spanning all the drives together as non-RAID, since that is a really bad idea, though possible.  While it eliminates the need to manually distribute the data across multiple volumes, the loss of one drive loses all the data.  Even if you have a backup, it can be quite a task to restore it all.

Yes, the RR2304 can be configured to do that, and quite cost effectively.  You can use the space on each drive for iSCSI or you can have the individual files accessable via SMB or other protocols.  It all depends on what works best with your Retrospect instance.

If you have room for it (it's very deep), you might also want to look at the RR2312 and start out with it's 12 bays at under $1000.  If you decide to go with RAID, a 12-bay system is more efficient in the use of the extra drive(s) needed for that.