Usually when it plateaud, it was when backlinks stopped appearing. So I would focus on marketing and link building.
When a site becomes flat, it can assist to take a step back and look at both the content and the structure. In a lot of cases, getting things moving again can be as simple as enhancing internal links, updating outdated content, and tightening up on-page SEO. Also, looking at technical things like crawl problems or page speed often reveals underlying problems. I suggest that you make small changes and keep a close eye on the results so you can see what is actually causing growth.
I’ve seen this happen after the initial growth phase. In my case, updating and expanding existing content, improving internal linking, and fixing keyword cannibalization helped regain momentum. Small technical improvements (speed, crawl cleanup) also helped, but better content depth and intent alignment made the biggest difference.
Adding fresh content twice a week and getting more high quality guest posts and backlinks will restore the site back to each normal ranking (or even go a bit higher).