Yes, it does it all at once. Even if the import crashes mid-way, it will roll back any incomplete data.
Done!
Maybe there are things which can be shared between the two, but it should be relatively straight forward. There are already things in place to make file-based migration easier.
Do you think the workspace projects should be top level projects or nested under a “Migrated from Trello” project?
The problem here is what the quote in that line of code in the trello import says: Attachments can be more than just files, in that case they may still contain a url but not of the downloadable kind. I haven’t looked into it yet but I guess there isn’t really a way to make that distiction when not using the mime type.