That’s for people who don’t know what system administration is who need support in making it work. For a large forum you’ll want at least a 2 container install, or maybe RDS and Elasticache instead of rolling your own.
If you want to launch with a Dockerfile you’ll need to use launcher to build your container and figure out a way to manage database migrations and precompiling assets.
If you know how to use Google for transational email, that’s fine. Most people don’t and we don’t want to teach them.
A large scale migration for an old forum typically needs lots of customization. Whether to run your migration in a development instance or in a production instance inside of docker is a matter of taste. I used to do it in a development instance but run all of my migrations in docker these days. Figuring out how to get all of the various pieces installed can be frustrating.
Do you mean “discord” or “Discourse?” Or maybe you want Configure Discord Login for Discourse?
Whatever you do, people hate change. I don’t think that trying to make Discourse be like vBulletin helps. If you want your forum to behave like vBulletin, then just keep using it. There are lots of themes that do lots of things. There isn’t really a guide except Search results for '#theme-component' - Discourse Meta. You can wade and search there.
One thing that I think someone missed is that though the first time you run the script it will take days, subsequent runs are faster because they skip the already imported data, so the final run won’t take too long.