FYI, since your system uses a Fusion Drive, you may want to perform a secure erase on the internal hard drive so that all your data is overwritten so it cannot be recovered. Unlike an SSD, the hard drive will still retain the data when performing a simple erase using Disk Utility....the data can still be recovered by using a simple data recovery app. You can securely erase the hard drive with Disk Utility by erasing the whole physical hard drive & the "secure erase" option to write a single pass of zeroes to the entire hard drive. The SSD portion of the Fusion Drive does not require this secure erase due to how SSDs work. The "secure erase" option in Disk Utility may be a button "secure erase", or perhaps "Options", or "Advanced", etc. ....it has changed over the years & with later versions of macOS the "secure erase" option will only appear when a hard drive is selected.
Another option would be to create & use a bootable ShredOS USB stick which can write zeroes to the physical hard drive. The easiest way of creating the bootable ShredOS USB stick using the downloaded ShredOS .img or .iso file as a source for Etcher (Mac/Windows/Linux).
FYI, here is an Apple article on all the steps that should be taken when selling or recycling a Mac.....Unfortunately for erasing the system, the article assumes the internal drive is either encrypted with Filevault or is an SSD. The secure erase option & ShredOS options I mentioned cover the other cases when a hard drive is involved such as in a system with a Fusion Drive such as yours.
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
