Resize Synology ext4 volume

Hi, I hope someone can help me here.
I have a volume that was created on my Synology DS1812+. It is a ext4 volume but limited to 16TB.

I have all 8 bays filled with drives (5 * 4TB and 3 * 3TB) about 22TB with raid 5.

I have created a second volume with the remaining space (about 6TB) as btrfs.
I would like the move that data from one volume to the other but as you can see I don’t have enough space free.

Can I move as much as I can from volume 1 to volume 2 and then ssh into the Synology and shrink volume 1, expand volume 2, move the data and repeat this until the data has all been moved ?

Synology tell me this can’t be done and that I need to back up the data, but I have no way of backing up 15TB of films and I really don’t want to be ripping all my dvd’s again!

Thanks in advance, Richard