I'm sorry to hear these things... and sorry for you and your family. I posted here in 2018 when Maya passed... so you can see that I empathize with you.
I think you'll likely second guess yourself quite a bit, I know I did. The realization that helped me stop the second guessing was quality of life. When the bad days become very bad, and the good days few and far between, that's when the quality of life question becomes the deciding factor I think. For Maya, it was mercifully quick in a way, as she was bleeding from her mouth and couldn't stand at all for more than a few seconds; you guys will have to make a tougher call I think.
As time passes though, you'll only remember the good things/times, and you'll be able to force these bad memories from your mind (the ones you already have, and the one that is coming). My wife always says that they (dogs) give us uncounted good days, and one truly horrible day (that may be a quote from someone else as well).
I used Andy James' "Gone" to say goodbye to Maya... still struggle a bit when I hear that song.