Growing up, I had a dog that was part Wheaton Terrier and part Retriever; he had the playfulness of a terrier and the sweetness of a retriever. He got a brain tumor, too. He started snapping at bugs that weren’t there, than having siezures, then started having difficulty walking. I was the last hold-out on euthanizing him in the family. My sense had been he didn’t want to leave us. One day we were having dinner outside on a 2nd-story deck, and he had been down below, and wanted to join us; he tried to climb the stairs which was far beyond his ability. That seemed like *emotional* suffering for him not to come up the stairs. (We all went and ate down there with him.) Finally, I agreed to have him euthanized.
We found a very sympathetic vet that agreed to come to our home to put him to sleep. We gathered around him, and he held his head in my lap. He gave a light yip with the injection, started thumping his tail on the ground again, then he stopped, lowered his head again as we all pet him, and fell to sleep.
I think that’s what we gained by putting him to sleep: he died while he was being loved by all of us.
Your wheaton/retriever mix sounds wonderful. I bet that was a smart dog. Some mixes are just amazing, very brainy, very healthy.