Are you sure you have no open transactions behind (DBCC OPENTRAN)? What is log_reuse_desc from sys.databases? Not sure about the names, I'm writing from the head.

These are really huge log sizes.

Loop should not bother you, because you are processing data in batches, not row by row. My MVP colleague Dean would ask "How would you eat an elephant? One by one meal until you are finished."

Other things that comes to my mind (other than avoiding log growth with checkpoints, or bulk logged recovery model) is to create additional log file to fit this operation, and drop it afterwards.