This option lets you downgrade to the OS your Mac came with: “ Reinstall your computer’s original version of macOS (including available updates): Option-Shift-Command-R.” https://support.apple.com/guide/mac-help/reinstall-macos-mch...

You can download the Mojave image from here afterwards: https://support.apple.com/en-us/HT211683

Backup your data before doing a clean install.