Here's an update:
Last Monday, Sept 5, Shrink was transferred to my QT. He was treated with a reduced dose of BettaFix (LFS convinced me that it was better than salt) and daily 100% water changes. All week he proceeded to ruin his fins, but didn't look infected.
Friday, Sept 9, I removed him from the QT, as I was sick of seeing him continuing to rip chunks out. I put him back into his home, with reduced water, so about 2.5 gals. He's now on a water change schedule of 50% every other day, with no medicine.
The next morning I started to see bubbles over the surface of the tank. By Sunday evening he'd made a small bubble nest in one corner. So he's much happier, back in his home. There have been no new chunks missing.
My only concern is the biggest hole, the closest to his body, looks a little red on the edge. I guess I'll have to keep an eye on it. I really don't want to put him back in the QT unless I must.