How do I insert tens of thousands of rows without locking?
You cannot get rid of all locking.  However, there are ways to speed up what you are doing:


Use multi-row INSERTs.  100 rows at a time will run 10 times as fast.
As mentioned, switch to InnoDB using pt-online-schema-change.  Then put multiple INSERTs into a transaction (BEGIN ... COMMIT).
The 6 threads could write to 6 different tables (such as CREATE TEMPORARY TABLE), then INSERT INTO real_table SELECT * FROM tmp_table.  Even on MyISAM this will run significantly faster because of 6 locks instead of 6*50K.  Read about concurrent_insert to see how to avoid 50K table locks into the tmp tables.


Please show us the SHOW CREATE TABLE.  There could be significant other issues, especially with indexes.