Moving just MacOS from HDD to SSD
No - that won't be easy at all but it's exactly what Fusion Drive was engineered to do.

What would be easy is to make a backup of all the files you care about - preferably using Time Machine and a second clone copy if you're not sure Time Machine will work for all your files as a just in case.

Then delete about 30 GB of space so you have room to get a clean installer and make a bootable install drive with the OS of your choice.

When you boot from that installer - the trick will be wiping both the SSD and HDD entirely - then getting it back into Fusion Drive configuration.

Now that you have an OS and user data on Fusion arrangement - loading from your backup (either in the Migration Assistant portion of the new OS setup or running Migration Assistant) would get you the SSD boost without having to implement your own dual tier storage system.

Since most of these specific tasks are already asked / answered here and would be far too long for one answer - here's the outline and I'll try to link to detailed questions on each:


Backup via Time Machine 
Optional backup using a clone (Disk Utility / Carbon Copy Cloner / Super duper)
Clear space so you have 30 GB free
Download the OS installer you want 
Make a bootable USB installer drive
Boot from the installer (or Internet Recovery) and erase SSD and HDD entirely
Then form an SSD/HDD Fusion Drive before installing the OS
Install the OS on Fusion Drive
Restore files from the backup