Wild idea:

If you can't create adequate indexes on this table, try creating a new table with the same structure but proper indexes (probably clustering on TestDTTM) and load the data. You'll need to do incremental loads, to prevent the problems with the log size, probably using a different tool such as SSIS or bcp.

Once both tables are synced, you drop the original and rename the new one, you might need to work with dependent objects as well.