Hi Andrew... thanks for the question. The Detailed Billing Report (DBR) can become huge as you've noticed and we are now recommending not bringing it into Apptio in it's native .csv format. To address that size issue and to provide reporting at the daily and even hourly grain, Datalink introduced a new approach to working with the DBR. The Detailed Billing Report with Resources and Tags (automatic transform) option you reference is the manner in which you can ingest the DBR, get new daily/hourly cost reporting, and avoid the issues introduced by the large DBR sizes.
At this point, the DBR ingested via this approach will not be available in TBM Studio as Apptio leverages a new file format to enable rapid and dynamic queries even with the fine-grained detail and large file sizes. As a result, the Cost Allocation report is still the primary source for monthly billing. I would recommend speaking with your reseller to get you access to that file.