Upgrade Error: 3.5.10 to 5.2.6

I had an issue after running an upgrade from 3.5.10 to 5.2.6. I believe it was an error with an incompatability with Structure and EE. So, I tried upgrading structure but that didn’t work. So, I then tried rolling back my files and database to try again. However, now the installer won’t run again. I reverted my config file and everything, but the installer won’t initiate. Now when I overwrite index.php, admin.php, system/user and themes/user I get the attached issue when I go to my control panel.

I feel like I’m missing something simple. Any ideas?