Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
I assume you clicked "Test Connection" and saw a success? You're right in that "The stock item was unable to be saved. Please try again." is commonly related to ElasticSearch issues.
What do you mean exactly by CSS/JS not loading? 404 on storefront for the assets? The html source isn't including these assets? They're not on disk? etc. etc. etc. Also what's your PHP version?