Avoiding temporary tables while sorting by a column in a different table
You may need to bump up the following variables as well:


tmp_table_size
max_heap_table_size


If these are too small, the tmptable goes to disk quickly.

If these are too large, the tmp table goes to quickly when the limit is surpassed but creates intermittency due to moving the large in-memory tmptable to disk before completing the tmptable's usage.

So, you need to perform a tight balancing act with these tmp table variables. These variables also have the same per-thread constraint that @DTest mentioned in this answer.

BTW in theory a sort buffer is a tmptable in itself, though governed by a different session option (sort_buffer_size). Since you are sorting with a group-by column and not a pure table-established column, temptables are somewhat unavoidable.