Synology formats each disk with DSM so its not going to read WD disks, I moved from a WD to Synology and had to reload data from a backup. If you can’t power up your device, I’ve read (but not tested) that you can connect all disks in the raid to a PC and mount your volume there, easier in linux than windows. There are many youtube videos to that effect and I hope it works for you.
DS920+ personally makes me uncomfortable with one of the RAM slots welded, a minor failure there outside warranty may tank the device. Unless you use transcoding, 1621/1821 may be a solid choice at a higher pricepoint though cheaper per bay and come with better AMD processors, ECC RAM, shielded case and internal PSU, a much better value proposition overall. All Synology units have the same DSM software which is generations ahead of WD.