If this still applies, or can help someone else who stumbles upon your question: yesterday I recreated my Fusion drive using Terminal. Apple support has a page about this that worked for me: How to fix a split Fusion Drive - Apple Support
Obviously, you should back up everything before reformatting your drives. I personally use Time Machine plus the online service Backblaze.
In reading another MPU forum discussion, it sounds like folks recommend running an external SSD to run the OS — if your Fusion drive ever starts slowing down too much on you or failing.