Very tough thing to handle...I’m so sorry you are going through this.....
Sasha, my Rottweiler of 12 year’s, helped me get through raising my ( then young) daughters, when I was a single dad years ago. Cancer took her fairly quick, and the vet came to my house to put her down....I had her head in my hands and she was looking right at me when she passed. I cried for ( what seems like) an eternity. Her collar hangs in my shop, and the Christmas ornament we had for her hangs on the tree every year, bringing tears.
As others have said, spoil them , take pics, but ultimately do what’s best for them, not you. Minimize their suffering will help you come to terms, knowing you did the right thing. You will know when it’s time....
A miserable experience....I wish you the best, moving forward....mark