AWS Elasticsearch as a primary database

As always, it depends.It depends on your requirements for the data store.

1. Are these transactions coming out of one of your own systems that stores the data and so that data is easily repayable if your index was to get corrupted or if you wanted to reindex / change the purpose of the data? Of course there are backups that you can leverage but if you are getting so much data in you would lose data if the index did get corrupted.
2. What other things do you want to do with the data? is it just there to search? Do you want to aggregate or join it with other data? run reports on it?

I agree with @Val (in the comments) and would not recommend elastic as your primary datastore, have a read of this for some more good advise. But in the end it depends, elastic search is a great place to put log data for example. Have a read of this and this for more advise on what elastic search is useful for.

I'm interested to know which direction you went with this (given your question was 3 months ago) and if you support or regret your final decision. Do you have any good learning for us?