Tail Mutilation :(

Gusgus is about 19 years old. I've had him about 10 years and he's always been an on again/off again fur biter. About 4 years ago I left him with a babysitter and ...he got heat stroke. I came home after a week away to find him bright red, on his side and with a bloody tail. Took him to the vet, got him taken care of but the incident caused him to start chewing his tail. The vet gave him anti anxiety meds which cleared up the tail biting after a month or so.
Fast forward to last December, he out of the blue began to chew his tail again. It horrible, bloody and the bites are very deep. I've taken him to see 3 different vets about this and the end result is a big idk. We tried the anti anxiety meds (no luck), treated for mites and pain. Tried the cone but the minute it comes off he's back at his tail and at some point he needs to be able to chew his toys or he's going to have teeth issues next!
Bottom line is he's hurting himself daily with no end in sight, the vets aren't helping and he's dang old. He sleeps more, misses jumps, falls off his shelves etc. I need a second opinion from another chin lover, but would euthanasia be in his best interest at this point? I check for infection every day, keep it clean and treat with a topical antibiotic but this has been bad for 6 months now...