Table Partition Function Empty Partitions
I can guarantee that no data older than 2019-10-01 will be inserted to
  the table, and I plan to keep SPLIT partitions when I am reaching
  2022-08-01


As long as no existing data qualifies for the new partition boundary range, no data movement is required for the SPLIT.


  I also plan to regularly TRUNCATE old partitions and MERGE the old
  partitions range.


Similarly, the merged partition will be empty due to the preceding TRUNCATE so no rows will need be moved to accommodate the new boundaries.