Just a quick follow up on the "garbage" I mentioned above. Two folders I clean fairly regularly are:
C:\Users\[main user name]\AppData\Local\Temp and
C:\Windows\Temp
Both of these tend to house files which get used in one session and never get cleaned accept by the user, so they can get fairly big over time.
Having just done my first Win10 update (from 1709 -> 1803), Windows leaves its old version on the drive (30+ GB). I just imaged that drive with Macrium Reflect, and although it took twice as long, the image is the same size, so apparently Reflect doesn't copy that with default settings. To me this is not paramount, since I am on 1803 without issues, but to actually save those would require tweaking Reflect to image them.
To recover that space, you need to run "Disk Cleanup" on C: and select "Clean up system files" at the bottom. This exposed 1.54GB of "Windows Update Cleanup" and 29.7GB of "Previous Windows Installation(s)". Jim mentioned this above, but I didn't realize the size of these until I got to see it for myself (that is a hefty percentage for a small SSD!).