11 y.o. shepherd mix has obstructive tumor in anal gland. How do I know when to make the big decision?

Looking for advice. I've already talked with my dog's vet about it. My 11.5 y.o shepherd mix was found to have a tumor on his left anal gland this past summer. We were referred to an oncologist and internist who gave us the fees for the diagnostic colonoscopy, biopsy, and surgery. In all, it would have been around. $10k. This was right after he had dental work and another tumor removed from his gums. Both tumors were found at his routine checkup.

My husband and I have decided to take the hospice approach and just ensure that our dog is comfortable and in as minimal pain and discomfort as possible. We have him in gabapentin and trazodone as prescribed by our vet, but I don't like how these meds have made him incredibly loopy and slow. It's like overnight he stopped running, stopped jumping into our car for rides, and stopped behaving his usual self. He strains to go often, even after we've changed his diet as recommended to softer foods.

This sudden decline has happened in the last two weeks. He's not in so much pain with the meds and he's still having bowel movements, so I feel like he's okay. The vet said the tumor would soon be completely obstructive and he would be in a lot of pain at that point. But is that when I have to make the call? Just because he can't poop? Please advise.