Investigate other ETL approaches
We should look at other ETL frameworks (both PHP and otherwise) and see what good ideas might be applicable to soong.
Let's treat this issue as a meta and open a separate issue for each library/framework/platform investigated.
Edit: On second thought let's make a list here of PHP ETL/migration packages on Packagist, and non-PHP frameworks. In most cases, a comment here and striking out the package/framework should suffice - only create a separate issue where a given package/framework has some particularly interesting ideas.
Note the above come from searching packagist.org for "etl" (plus PortPHP suggested by @weitzman in soong#8 - I've also searched on "migration", but at least among the first couple of pages of results they look pretty much to be about database migrations (e.g., changing schemas between different versions of a framework) and not applicable to this project.