Factory reset halfway through filevault decryption?
The secure way to do it is to simply erase the dive, don't decrypt it first. That makes recovery of your data impossible - whereas if you decrypt it, there is a possibility it can be read later by a bad actor.
See Apple KB - What to do before you sell or give away your Mac
You can wipe it without decrypting. Just use disk utility first when you are in the installer system and format the drive you are installing to.