How to stabilize baseline traffic through SEO

My site traffic still has a downtrend. Going up to around 1.5K/DAY page views when social media posts are made, and down to less than 100 over the next week or so. Not all posts generate 1.5 K traffic though. Only some. Trying to do the following: 1. Analyze why some posts perform much better than others. 2. Raise the traffic baseline without constant posting. I have the basics of Google Analytics and GTM set up, as well as the target web pages indexed by Google and lit up in green on YoastSEO, but the majority of the traffic is either direct or organic social, without much coming through organic search. I heard some SEO folks can do on-page SEO where certain keywords are leveraged to help with search engine ranking through relative content, but I'm not sure how this works. Simply post relevant articles and wait? If you have some experience or can offer some advice, please let me know.