It looks like you’re going to have to pull out the cast off row if you really want to make the ending looser. I suggest a knitting needle or some sturdy sort of thing like a chopstick or pencil so that you don’t lose sts as you pull out the cast off. Then you can take out about 10sts, put them back on the loom and cast off more loosely. You can pull it bit of extra yarn into each loop to keep it loose.
As you come close to binding off the last sts, undo 10 more sts and put them on the loom. Repeat the cast off.
The only alternative I can imagine is to pull quite a bit of extra yarn into the last stitch bound off and then work that yarn into each stitch until you reach the first cast off stitch. This sounds difficult and tedious though.