My suggestion to those looking to import very large amounts of data via csv is to do it locally (If you are setup on a local machine using wamp or xamp, etc). You can increase your server resources in the php.ini file to avoid all of the time out errors on upload. Then export your site and upload it back your web host.
I just uploaded 50k rows of csv data in one go and it completed in about 2.5 hours. This page has more info on maxing out your settings to avoid time out errors. https://docs.appthemes.com/support/how-to-import-very-large-files/