A few things.
One, SSDs do not need defragging, which is a tool designed to organize the physical layout of a spinning hard drive so the read head can find them more quickly on the disk. This is irrelevant to an SSD. Even on a spinning drive, the Mac OS defrags all files above a certain size, 10MB IIRC. On a Mac at least (don't know about Win), the OS moves things around to different memory locations to maximize the service life of the drive, so 'defragging' may actually be detrimental to the life of the computer.
Two, I skimmed the thread and didn't see a mention of which Mac you have but I am assuming it's a laptop. Some have the "SSD" soldered directly onto the logic board which is not upgradeable or replaceable. Most current models aside from the new MacBook and the 11 inch MacBook air have SD card slots on them. There is a brand of SD card called 'JetDrive' which is a slightly non-standard shape so that it sits flush with the edge of the computer when inserted. B&H sells the 128 GB for $65 or 256 GB for $165. This provides a big storage boostnfor a small cost. Read/write speeds are not as fast to/from an SD card as an internal SSD to my knowledge but that shouldn't matter for playing music or video files, only for large files that you need high speed access to.
Lastly, a common place for storage space to disappear is in the Downloads folder. A lot of applications leva assorted debris like installers there, for example Microsoft office whose installer is about 1.5 GB. Many of these installers make the problem even worse by creating package files from the disk image (installer) files and leave them behind too,which doubles down on the problem. If there are any .dmg or .pkg files in your downloads folder, and those apps are installed on your computer, you can safely delete those files.
A great way to figure out where your storage space is going, including those annoying 'Other' files, is a $10 application called DaisyDisk. It gives you a visual representation of everything that's on your computer files and delete them. Word of caution, as a general ruledon't delete anything in a 'Library' or 'System' folder unless been okayed by someone along the lines of AppleCare. Lastly, if you have AppleCare on your computer, definitely take advantage of the over the phone technical support. They can help you sort all this crap out.
Hope this helps.
