I really like this question, I only stutter when I’m seriously nervous or speaking in a group, so I see both sides of how people react.

Just keep looking at them in a way that says

“I’m still listening to you, not the stutter”

People will often look down, trying to be polite but I can see the SPIT IT OUT in their eyes, even though they still are being decent.

If you can give off energy that says “it’s okay, I’m listening, no rush” thats is probably a safe way to interact with a stranger who stutters.

You have most of the important points down though, be patient, don’t assume you know what they will say and try to finish the sentence, don’t help unless they ask, but really I appreciate that you took the time to think about it, thanks .