Hello @Chris Andre Buys !

Wow, 400Gb of media...

Frist of all, you may enabled the pruning for the non-active accounts to clean them with their content. It is possible to do in the Studio->Settings->Account area:

The unused media can be found in the Manage section of the every media module, just clean all there without author.

The content of the cache, cache_public and tmp folders can be clear before migration - just leave there the .htaccess files only.