Is it time to euthanize over aggression?

We have a 2 year old cocker spaniel. We got him as a puppy and tried to socialize him as much as possible. However, he is still aggressive. The ONLY people he will let around him is myself, my 8 year old daughter, and his groomer/petsitter. He wears a muzzle to his vet visits. We have tried 2 different dog trainers. He bit one trainer within 5 seconds and she wouldn’t train him after that. She said he might have mental issues. He also bit our neighbor. I had him on the leash but he got to him before I could stop him. We no longer have him around people. He is in a crate whenever we have guests. We also tried medication prescribed by our vet.

The latest bite was our daughter. He bit her on the finger while she was putting the leash on him. He has never shown aggression to her before.

I feel like my only option is to euthanize because I can’t rehome him. I just feel horrible about it and my daughter will be devastated.