On my iMac, I'm attempting to wipe my Fusion Drive. I receive the following error when I erase "container disk3": The volume “...” couldn’t be unmounted because it is in use by process 673. Alas, the Erase button on the Fusion Drive is greyed off. I can see the following on the side panel: Fusion Drive > Container disk3 > Macintosh HD > Macintosh HD - Data > Bootcamp. In this scenario, what should I do? In addition, considering that I have a Time Machine backup from Catalina and a backup of my files on an external hard drive formatted in MacOS Extended Journaled, which format should I choose—APFS or MacOS Extended Journaled?
If you want to erase the BootCamp partition, you ought to boot into the macOS Recovery Mode, launch Disk Utility, and make sure to select the base of the Fusion Drive (called "Fusion Drive") and erase it as a GUID partition and APFS (the top option). After that, you can restore from your backup which will use the whole Fusion Drive.
Here is how to boot your iMac into macOS Recovery Mode: How to Use macOS Recovery Mode on Mac (M1/M2/M3 Mac)?