Christophe,
Welcome. Many of us using boot SSD of any type have a larger documents drive and a smaller boot/applications SSD (be it 2.5" form factor, M.2 form factor, etc.). It is easy to fill up a lot of drive space with "documents" be they music, photos, videos, etc. Are you sure that you don't have a lot of such files sitting idle on your boot drive?
There are easy-to-find methods for shifting the automatic save-to library for programs to your documents drive rather than your boot drive.
I have been using Acronis True Image 2014 and 2016 as a method to clone from a HDD boot drive to a SSD boot drive. I use an external eSATA attached drive as the target for the Acronis image to be saved on to. Once that is done then I can clone back on to a larger or smaller SSD and the program upsizes or downsizes automatically to fit properly to the new drive.
You'd want to set up your original HDD and the documents HDD so you shifted the libraries (My Documents/My Pictures/My Movies, etc.) over to the documents HDD before this process, of course.
We have some pretty powerful workstations built like this but never ever get anywhere near what you have on your boot drive right now.