RCSI & ETL processing compatibility
With Snapshot or RCSI isolation you get optimistic locking model where Readers don't block Writers and Writers don't block Readers. Beware that with optimistic locking query results for Readers may differ from pessimistic locking, since Readers don't wait for Writers to complete

Would Snapshot or RCSI isolation help when performing the insert/update ops in Step 3?

Writers still block Writers since Exclusive locks are held, but it of course depends which rows / pages / tables are locked. So no, I would say RSCI or Snapshot won't help for step 3
In fact, Snapshot isolation may potentially even make things worse, since it operates on transaction level while RCSI operates on statement level

What are the risks of introducing update conflicts?

According to the table below, you may get the update conflicts in Snapshot isolation (not RCSI)