We just put 2 of our 3 dogs down about 6-8 months ago. Granted, 1 dog had a brain tumor and the other cancer. But both were quality of life. Unfortunately with the brain tumor, we had only enough time to make an appointment with an in home euthanasia (about 5 or 6 days) and put her down. The other with cancer, with treatment, he passed after 6 months from diagnosis.
Our 3rd dog, she is 13 (pitty mix) and she spends most of her day sleeping on the couch or in bed. She has some of the same stuff u mentioned OP. Lumps, bumps and arthritis. None of the lumps/bumps are cancer and her arthritis she's doing fine with medication (1 pill a day). Since she was 12, I have taken her to the vet every 6 months abd make sure everything is fine. Blood work, urine/fecal, x-rays (her lower back is where the arthritis is) and everything still comes back fine.
We did have a 4th dogs years ago. (Lab/pit mix) that lived to be 15 almost 16. He is the one that wr should have put down probably a month b4 we did. He's quality of life was crap. He was bumping into walls/ not eating drinking much, lost control of his bladder, ect ect. But he was my wifes first "big girl" dog and we weren't mentally prepared to lose him yet. But the day we put him down we felt horrible cause we let it get as bad as it was.