Betta Fish: Ethical decision to euthanize or special care?

I want to preface that I’ve had Betta for 30 odd years and never had anything like this happen to any of my betta fish.

I’ve had this beautiful boy for a week and a day. He was one I waited a month to be posted online and then bought him, waited another week before he finally arrived.

My tank is 6.8 g shallow, weekly water changes, and water parameters are Ammonia 0ppm, Nitrite 0ppm, Nitrate 15ppm, Kh 71.6ppm (4drops per 5ml), Gh 214.8ppm (12 drops per 5ml) Ph 7.5, TDR 389, and temp is at 79°.

Betta shares tank with Pygmy Cory and Neocaridina Shrimp.

Three days ago while doing a water change my betta jumped out of the tank, somehow I didn’t notice until I’d finished, then realized that I couldn’t see him, and found him near the back corner of the tank. He was still alive, but tacky/sticky and he had somehow broken his left fin in half.

I put him back in his tank and turned the lights off, added a few drops of SeaChem Stress. He was gasping and fairly still but swimming a little. He seemed to be doing better the next morning but still in rough shape. The next day, I found him again, behind his tank on the floor. I thought he was dead because he was dry, crescent shaped and his eyes looked foggy. But when I picked him up he flicked his tail. I put him back in the tank, and he was swimming but definitely struggled. He pretty much laid down on the sand and didn’t move unless disturbed.

He just looks horrible now. His tail and fins essentially dried up and melted off. I’m worried he was just oxygen deprived and won’t recover, but I know fins can eventually grow back. He doesn’t seem like he’s in pain so much but maybe that’s just me hoping? Idk what to do. I don’t want to give up on him but I don’t want him to suffer needlessly. And selfishly I spent a lot of money on this fish and I’ve only had him a freaking week and I’m annoyed and upset.