Sorry to make such a mundane suggestion, but have you tried moving ~/Library/Preferences/com.apple.Safari.plist to your desktop, etc...?
As a matter of fact, I had not tried that, so 'twas well worth the effort. That I did not achieve success is still worthy. At least that is one more possibility that can now be ruled out.
Harv 27" i7 iMac (10.13.6), iPhone Xs Max (12.1)
Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
