|I think you are being overly complicated....; and possibly misunderstanding what an NAS is for... An NAS is for storing/sharing files across a network; and whilst the NAS file format is typically ext3 or ext4 (Linux formats) that isn't an issue over a network. (File formats are - abstracted - removed from the process over a network.)|
An ext3 or ext4 drive can be accessed - read certainly (and I think also written to) with suitable add-in utilities for Windows and Mac. Otherwise one must use a Linux OS to access (read/write to) them.
A typical NAS can be two or more drives; and many (all?) use a Linux based OS to run themselves. Certainly the QNAPs range do.
The most basic (minimally safe) is a (two drive) Mirror system RAID-1. The two drives are identical. RAID-0 (again a two drive system) stripes data across both drives; and if one fails. then all is lost...
So looking at simpler arrangement to that which you seem to envisage...
Perhaps use a two drive system in Mirror configuration - which means both drives are identical to each other; and if one fails the remaining one can rebuild the Mirror to the replacement drive. Either drive "can" be removed and read/written to via Windows or Mac OS using a suitable utility.
If concerned about possible failure of "both" drives at any time... then simply attach an additional external drive - via usb - to the NAS; and backup the server to that drive. Equally - and takes longer - copy the contents of the NAS to that drive.. - and use ntfs format for that additional/external drive. That external drive can then be connected (via usb) to any windows system and read/written to. For Mac systems you may need a another add-in ) if wishing to write to it...
Some reading about NAS and ext3/ext4 etc... and accessing the file system:
QNAPS have decent kit; and likely they have usb3 systems etc. I have an older QNAP Mirror system; been very happy with it. When a drive did fail (not QNAP's fault) they were top marks in support re' setting up the whole system afresh - to enable the good disk to rebuild the Mirror on the new drive. This was done by their tech support chaps via Team Viewer.
Any system is only as good as the drives installed of course... Ensure yo put in drives that are designed to be left on/running as it were for prolonged periods. Many NAS are delivered with drives more designed for short term use; as in a typical laptop/desktop etc. There are drives designed/spec'd for server us.. WD have three models and I suggest that Seagate etc. likely are similar?
Others here with more experience with NAS (mine is somewhat limited thes edays..) may offer other approaches; so watch this space?