I suspect there will be more to the Sue and Don chapter yet to come. Afterall, the level of Sue's emotional attachment to Brad was only revealed to STB well after the after they stopped sleeping together. I suspect the same thing with will be forthcoming about Don.
It may take Sue some time before she seeks a new FB. Unlike the planned ending with Brad, there was an abrupt ending with Don that may take more time for her to process, grieve, and put behind her.