I’m not saying this is what might have happened to your dog, but I had a lurcher bitch with the same symptoms.
I couldn’t see what was wrong so took her to the vet. They wanted to do blood tests and x-rays, but luckily the boss vet, who I knew, came in and had a look.
He felt around the dogs throat then a look down with a torch.
He got some long reach forceps and retrieved a rabbit rib bone that was lodged across her throat.
Immediate and complete recovery!
Second opinions can’t do any harm.
Hope the dog makes a complete recovery.
Cheers.