I want to be able to retrieve our data as it has family pictures etc. and a lot of other valuable data stored on it.
You won't be able to mount the individual drives because Synology employed (as a default) a technology called SHR (Synology Hybrid RAID). So, to recover anything, you will need another Synology unit. Short of having a second Synology unit, your data should have been backed up.
SHR is an automated RAID management system that makes storage volume
deployment easier than traditional RAID systems. SHR will allow users
to handle RAID management, expand storage, and maximize storage
capability even if they do not have a fine grasp of various RAID
levels. SHR allows for 1-disk or 2-disk worth of redundancy - meaning
the SHR volume can suffer up to two disks lost, and the data volume
will still be available for use. Note that a RAID volume (either
Classic RAID or SHR) is not a backup system.
Even if using "classic" RAID, you still woudln't be able to mount the individual drives; they would be need to be mounted as the entire array (and this is before you get to the filesystem).
It can't be said enough that RAID is not a backup. It's a fault tolerant device, not a disaster recovery device.
My suggestion is to obtain a similar Synology unit (secondhand via eBay is fine), rebuild the array then archive them to a USB drive.