How to erase Macintosh HD to use it for storage

I'm running macOS Monterey, v. 12.4, on an iMac (Retina 4K, 21.5", 2019) using as Startup Disk OWC EnvoyPro SX. The Fusion Drive was not performing (8GB mem), and after reading in the Forum, and suggestions, I got the OWC SSD - EnvoyPro SX - and the performance is supper aceptable. ;>)

In order to dispel doubts that Macintosh HD is intervening in processes, can I erase it, and use it for 'storage'? 2) Is that possible - it doesn't run, say, graphics? If possible, and having OWC SSD run the show, is there a process - a link - where I can read how to do this? 3)Any warnings?

I appreciate any suggestions and advise on this - thank you!