OP, our beloved Dane started to fail around the same age. The poop thing will likely eventually become a general weakness in the back end.
Start to come to terms with this being a decline, but also consider what you see the end looking like. We didn’t know “the day”, but one day, her whole back end gave out and there was no choice. For me, I’m glad to some degree it went as it did, because we had a wonderful day, and I didn’t have any option but to make an awful choice.
By breed standard, your wonderful companion has lived a good and strong life. It’s not going to get better, and if you choose to extend their life, you will have other choices, like making sure they never go up and down the stairs, etc.
There never a right time, or right answer. Our girl has been gone almost 2.5 years now, and my heart hurts still.
I’m so sorry, you’re going though this.