Little Bookmark Box works great and is simple.

Little Bookmark Box works great! I needed to replace a bookmark system as that one was not carried over by the developer to macOS Sierra. This app puts an icon in the menu bar, and you click on it and get a pop down menu which is a list of boxes. Each box can hold a single bookmark or a bunch of related bookmarks. The pop down menu can be situated where you want it, and you can hide the pop down menu when not using it. The result is a very nice system for holding bookmarks that you do not have room to include easily on the favorites bar of Safari. This way you can avoid a deeply embedded system of bookmarks coming off the menu bar.

gkols about Little Bookmark Box, v3.1