If OS and apps remain on old drive, then you can move only user files - docs, pics, videos, music.What is the best way to move all my other files to my new drive?
"All my other files"..............
Does that means only personal data files like pictures, documents, video clips, mp3 music, etc??
Or does that include some installed programs?
Do all of these "all my other files" exist somewhere within C:\Users\your user name?
Ideally I would like my M.2 to be only used to boot my PC and have Steam with a few games data there. Everything else ( documents, apps, pics) on my new SSD.
You are likely to have issues trying to keep Windows on one drive and "apps" on another.
What is the total capacity of the original drive?
What is the total occupied space on the original drive?
What is the total capacity of the new drive?
Is the new drive NVMe, M.2 SATA, or 2.5 inch SATA?
I assume you want to leave Windows where it is now.
I'd install the new drive, format it with a single partition, perhaps with the D drive letter.
Manually move whatever personal data to the new drive in a folder structure of your choice. Like D:\pictures, D:\mp3, D:\video, etc.
If you make a new file tomorrow named pork chop recipe.doc, save it directly to D.
All installed applications remain behind on the original drive.
That's what I'd do in your situation.