1. defrag is of value only on Hard drives.
It gathers up possibly separate parts of a file so they can be read with minimal mechanical motion.
On a ssd, there is no mechanical motion.
2. Depending on the defrag utility., you should be able to first do a scan to assess the possible value of a defrag.
Once you have defragged, it is unlikely that there is any benefit to doing it again.
And, if you have lots of free space, files on the HDD are going to be placed well in the first place.