In my experience, we got the green card in July/August 2021 and had the interview in January 2022.
We used the DC office.
I do not think you would need to travel too many times so it would not be worth the travel document.
I have not done this because my children live with myself and my wife.
I do not think this is necessary unless you there is a divorce/separation/death involved.
Thanks for adding your experience.
It seems that the grandfather is required to attend as per most of the posts.
My father was not required and that could possibly be due to COVID 19 restrictions at the time.
I may edit my original post to reflect that if more people post that the grandfather was required.