I am planning to do the same thing.

In the official docs, under limitations, here
https://docs.aws.amazon.com/dms/latest/userguide/CHAP_Target.Elasticsearch.html#CHAP_Target.Elasticsearch.Limitations

It states that

OpenSearch Service doesn't support writing data to pre-existing indexes.

My current workaround planned is to make use of index templates, and then have dms automatically create the index in opensearch, and have the index template applied to that automatically created index based on a naming pattern that you can define in the index template.

Not sure if it will work out yet, but if it does, then in that index template you could also configure your replica shard counts