Hari do you recommend meilisearch for my need?
I have a similar volume and when I tried Meilisearch it worked quite well. Queries were fast (few milliseconds) even on the 600k posts collection. I'd say give it a try, I think it's the best trade-off between cost and performance/features/quality at the moment. The only issue I've found is that indexing can be quite slow (much much slower than Elasticsearch) but I know they're working on it.
I personally didn't go for it because I found out that Elasticsearch is not that expensive in terms of resources as I anticipated, and it's much more customizable. Also, it provides an API to update documents in batch by query, something that is useful when you need to keep the search database in sync with MySQL.
Hari i am looking for full-sentence matches in post content search
Do you mean exact match of a sequence of words? Meilisearch by default takes into consideration proximity for ranking search results, you can tune that: https://docs.meilisearch.com/learn/core_concepts/relevancy.html
It also supports using double quotes "
to force and exact query: https://docs.meilisearch.com/learn/what_is_meilisearch/features.html#phrase-search
Sonic doesn't support phrase queries for example, while in Elasticsearch you would do that with the match_phrase
query.