Theoretically a 5 TB drive should never need to begin shingling the data until it reaches about 1 TB of data. That however is up to the drive firmware when and how it happens.
I suspect the “noticable breaks” you mention regarding rsync on ext4 is a result of buffering. Reading the data and reported write speeds are quick until the buffers are full then it drops to actual write speeds (on all media). If the total data written does not fill the buffer then the reported write speeds are the speed at which data is written into the buffer (which is usually much faster than the speed it actually is written to the media, especially for HDD)