OK, so the RAID device has the full capacity, if your backups are solid, you can try to manually grow your volume
Here is an old(ish) topic with step by step instructions on how to grow a stuck static volume
https://forum.qnap.com/viewtopic.php?t=173439
In your case it looks like md1 (the RAID itself) has already grown, so you should be able to skip this step)