Volume on RN4220 read/only - trying to understand btrfs

Going into read-only mode is usually a last gasp for the volume to remain intact.  A re-boot may well kill it completely, so that's not your best next step.  It is highly unlikely that you can fix the problem permanently short of destroying and re-creating the volume.  So, now is the time to make sure your backup is complete (and can be restored quickly, so probably not just on a cloud based system) and make plans for how and when you are going to do that on a production system.  You can try a re-boot then and see if it fixes anything, but it's unlikely.

Once you get this sorted out, you should take a good look at your backup system.  A second NAS that mirrors the main one would make it easy to simpy swap it in.  While Netgear has not implemented anything that makes that especially easy, it can be done.  I even have almost that at home, though I have two smaller NAS that each mirror part of the main one.  I just use periodic rsync jobs, but for somethingt that changes more often, lsyncd (https://axkibe.github.io/lsyncd/ ) would be worth looking into.