You can overwrite it with the original fairphone boot screen. You’ll need the manual installation ZIP, a USB data cable, the fastboot command line tool, a command prompt in the directory that you extracted the ZIP file to and the phone in fastboot (or bootloader) mode, i.e. hold the volume down button while booting.
Yes, for mac the command is the same as for linux.
The boot screen is in the manual installation ZIP. After you extract the file, it will be in the images subfolder.
Can’t say why the FP Open OS installation failed, you didn’t give any details. Normally I’d expect some message to be shown if anything fails.
In the past, sometimes people didn’t type “Yes” exactly as instructed at the prompt asking if one is sure to delete all data on the phone.