Fastest way to reindex from 2.3 to 6.4

Elastic Stack Elasticsearch
1

Hi
we have 300TB indices 2.3.5 and we want to upgrade to elasticsearch 6.3.

we set refresh interval to -1
replica to 0
request_per_second to -1
and we run 10 remote reindex API on 3 coordinator node.
our tests show that it will be converted over 3 month.
how can we decrease the reindex time?
any suggestion will be appreciated.

2

not use the default api.
use java client search the es 2.3 data and use bulk to write to the new 6.3 es cluster

3

we increase the bulk size and had better performance.