Home / Add ability to collapse all bookmark elements at the same level and their descendants
As the Manual page of this forum states, the daily and pre-release builds include the ability to use Shift+/ to collapse all tree items. I guess that ought to suffice.
Did you try Shift+Enter as well?
Personally I think this is a very niche feature that’s unlikely to be implemented, but like I said in your other thread, feel free to create an issue for it on GitHub and maybe the dev will agree to implement it.