I recommend using the OOTB tool if it satisfies your use-case (you can even do a metadata export of assets to "baseline" your CSV file, and update that). Nice thing about OOTB is if something doesn't work, you can log a Support issue and Adobe will take care of it 🙂
To be clear though, Metadata Import doesn't import the 3TB of assets, so I guess that # isnt really important for metadata update, more the # of assets being updated.
We have been using Step 1 for approx 30k batch, without any issues. Both the tools work fine. Usually an admin should be able to use ACS tools and general authors the OOTB metadata import option.