For anyone reading this post for solution, I am currently using OpenSearch for FullTextSearch with NextCloud using this project : - https://github.com/fairkom/nextcloud_opensearch. But I think this is a community project by fairkom, so you will have to trust it yourself and use it at your own risk as it is not in nextcloud app store.
I guess this will work with locally hosted opensearch too, but I haven’t tried yet.