As a fellow dog lover, I understand how difficult it must be to lose a beloved pet, especially when they are getting older. My last dog passed away a few years ago and it broke my heart.
I hope you can find solace in the fact that you did the best thing for Khan by making the decision to have him put to sleep. It's so hard to watch a pet suffer, but it sounds like you gave him a good life.
Yes, it is difficult but as you say it's also good to know that he was given a good life. I will no doubt get another dog at some point as the house feels empty without Khan. But I don't think I could ever rescue an older dog because saying good bye is too hard. Which is sad in some ways as most rescue centres struggle to place older dogs with a new family, probably because of this.