I am also skeptical when changes is making on the Faith because God does not change. When Mary had entrusted the rosary to saint Dominic she told him that the rosary is 15 tens, no more, no less. If Mary found it important to add another mystery she would have done it.
In addition 4 mysteries are very long, it encourages more to recitation than meditation, and the 3rd light mystery does not correspond to a specific biblical passage that can be contemplated to draw fruit, so this is not actually a mystery.
And to answer your question, thursday is for Joyful mysteries