This came up in active for me - so sorry, OP.
Not a dog, but we had to say goodbye to our best beloved cat last year (also cancer) and I am so grateful we were able to give her a gentle, peaceful end, surrounded by love. We actually could have prolonged her life with chemo - but it wouldn’t have cured her, and she would have been really distressed being carted back and forth to the vet for treatment. I’m proud of us for doing the best thing for our best girl rather than what we would have liked, her to live for longer. It is the last act of love for your pet.
20 years on, it haunts me that I wasn’t there for my childhood dog being pts, and while it was horrible seeing our cat go, I am so, so glad I stayed with her in her final moments.
This possibly isn’t helpful, so please ignore me if not or if it’s upsetting. But things I wish someone had told me. I had the vet take her paw prints, and a few weeks later had one of them tattooed on my arm. I am so pleased I did this (I saw it on TikTok of all places). I had to go out of the vet through a back door because I was so upset, but my DP had to go back to reception, pay, and collect her empty carrier. That was awful, and with hindsight I would pay upfront (although they did say we could ring later to pay over the phone). We paid for a private cremation, and then I was blindsided when the pet crem rang me asking about caskets - I was at work and about to go and teach a class, and again, with hindsight, I wouldn’t have shelled out for the fancy urn that I did, because I could have got something nicer/more personal on Etsy or similar.
Also possibly not useful, and not for everyone, but we adopted a little boy cat very quickly afterwards - he was a stray kitten, we have given him a lovely home, and he properly rescued me when my heart was broken. he’s an absolute joy. When we were in the specialist vet place discussing options for our girl my MIL was with us, and I asked her what she would do re chemo. She said that there were lots of other pussy cats who needed a home, and that letting S go would “create a space”. I am absolutely certain our girl popped him up on the Cats Protection website for me to see.