Well...there are a couple of ways forward
But you really really really need another drive, for multiple reasons.
Several possible ways to do this.
Theoretically, you could do it this way:
On the HDD:
Defrag
Shrink that partition to 600GB or so
Create a new partition in that now unused 400GB space
Move your Steamapps folder, and your pics/video/whatever to that new partition
Get that original C partition (incl Windows) to below 200GB
Then you can migrate just that partition to the new SSD.
Resulting in the SSD having the OS and applications, and that other partition having all that other stuff.
However...anytime you mess with partitions, you should have a good copy of whatever is on that, just in case. Far too easy to get things wrong and destroy all that data.
Or...(and this is recommended)
Just do a clean install of the OS on the new SSD.
Reinstall your applications, on the SSD
Steam games can be accessed where they are on the HDD. Origin as well.