Hello,
I think the very first question that we need to answer is:
How many records do you have in your SQL DB?
If it’s anything below 1 million, I wouldn’t even think about adding ElasticSearch into the tech stack.
As you’ve already figured out, there are many open questions and little documentation.
You can check out the sugar side of the docs:
https://support.sugarcrm.com/knowledge_base/installation_upgrade/installing_and_administering_elasticsearch/#Key_Metrics
https://support.sugarcrm.com/documentation/sugar_developer/sugar_developer_guide_14.2/architecture/elastic_search/
It might not apply 100% anymore, but the roots are similar.
If your initial index takes only a few seconds, most likely it’s an indicator that you don’t need ElasticSearch, due to the amount of data you have 
In a recent project, we had around to 700k contacts and plenty of other records in related modules.
It took close to two hours to re-index, if I recall correctly.
Have you checked in Kibana, whether everything looks ok?
As for the documents: I believe you’re right - haven’t seen that as well.
As for the UI:
As you can already see from all the replies to your topic and the many topics around ElasticSearch in the forums here
it seems to be a low priority.
I assume, that most of the SuiteCRM installations on earth contain a few thousand / seldom tens of thousands records, with its (current) B2B and little marketing automation focus.
So therefore, I believe there are many more pressing topics for the UI than the ElasticSearch integration (Campaigns, KanBan, Admin, Themes are just a few ideas).
Which leads to the requirement of a developer to get things integrated.
A dead end? Well - ElasticSearch is a great search engine / index and it keeps on developing like adding Vector capabilities.
Those help little with the standard SuiteCRM search, since this looks into the traditional index.
In several newer projects, AI, semantic search etc. are on the table.
Going forward, there might be two sorts of searches - your traditional SQL query and then a semantic search feature that can build reports, filters etc.
Elastic Search, I’m not sure - maybe before going down that route, I’d look into the business requirements and maybe take a CDP / data lake into consideration as well. Not the same benefits, but maybe more beneficial to a business with that many data records to search through.