I-Mac Hacked--Safari Issues AFTER OS Re-install

My Imac has been hacked. I've replaced the router and re-installed the OS (Big Sur) but someone continues to access my Safari browser to install adult websites in my bookmarks and favorites tab on safari. This is only noticed when I turn the mac on and load up Safari. Even when it's not connected to the wireless network! How is this possible and what can I do to eliminate the issue? (I don't believe this is being accessed via Apple ID as I've not entered into this mac, I've also changed my apple id and it's two factored).

Any suggestions would be appreciated. Thank you!