Long story short, my 3.0.14 PhpBB forum is half broken because it relies on multiple mods, several of them have stopped being compatible at some point. The visitors' side is OK, but the admin side isn't, several parts of the admin are broken, get me logged off and relogged into a white page until I clean cookies, my server's error log registers missing class declarations, the part about uninstalling mods now loads a white page every time. Fixing it by hand might be possible, but it would take a massive amount of time with a great risk of screwing up at some point.
Which is why I come to ask, humbly...
Please: is there a way to keep my forum's contents, the posts, the database, the user's avatars and signatures, and transition it to a VANILLA phpbb, fresh out of the box, as if just downloaded from phpBB?
I'm perfectly fluent in matters such as FTP/SFTP transfers, file and folder comparison/diffs to find changes and revert them, database sql/gzip exports/imports.
But I don't dare go full Leeroy Jenkins, delete everything of the files save config.php, the emoticons and avatars, upload a vanilla PHPBB, and hope it will work at once

Better ask

Please, if you know how to do it properly, cleanly, I will warmly welcome the help, thank you!