Need to completely reset Safari

Hello,

I've got a weird issue with my Safari version 12.1.2 on OS 10.12.6 Sierra. One of my online accounts has been corrupted and it is an issue with a file somewhere in my User account. I've reset Safari by emptying the cache and removing all website stored data through Preferences and this has not helped. I have also deleted the entire Safari folder in my User library and this has not solved the issue.

But if I log into a backup account I have on my MBP then things work fine. So clearly it's an issue just somewhere in the User files but somewhere other than the Safari folder in the User library.

Can someone tell me how to reset Safari so it truly gets restored back to a pristine state? Unfortunately I can't just switch users permanently.

Please help!