All sounds like sensible advice.
I might be able to crowd source a partial solution if I can convince some users to manually scan for and mark topics for deletion.
Right now the site on the domain is essentially a clean slate placeholder while a freelancer is fine tuning the import process from old forums (tweaking the import script for MyBB in particular, so that hopefully the custom user profile fields I had set up there will import along with users and their posts. Also hopefully parsing the MyCode in some MyBB posts properly (right now the formatting codes are visible)
I wasted a week fruitlessly attempting this import myself but just could not achieve a dev environment with all prerequisites in place that would allow me to do so from either a Windows 10 instance of Ubuntu or a DigitalOcean based instance, based on the official guide for use of the MyBB import script.
After painfully thrashing around, solving one error message after the next, I finally struck an absolute brick wall either way when it came to getting the SQL database to be accessible when executing the final ruby on rails command that would initiate the import.
Linux and Ruby both seem to be written by sadists, for masochists, both being astonishingly fragile and arcane. In such an environment, the odds of catastrophic problems when manipulating databases do seem high!