Why is my Website SEO and Ranking Declining?

I started working on a website's SEO in August, and the strategy's I used to build do-follow backlinks and adding more relevant content on the website worked out pretty well for the first 2 months, however in the past 1 month my rankings have dropped for some of the high volume keywords I was ranking for. What's changed in the past month is that I have stopped making free directory submissions, classified ads, local business listings etc.

The past month I only focused on High Traffice, Niche relevant Do-follow backlinks, however, they had little to no effect on my rankings. I got about 9 guest blogs with about 2 keyword-relevant anchor text from each guest blog backlinked to the product pages of my website. However, the rankings for those keywords didn't improve much.

The website I am working on is in the Natural Stone industry niche in New Zealand. The website doesn't showcase any pricing and hence all product pages are like portfolio pages. The website is based on WordPress and our website page speed is 90+ on both mobile and desktop in Page Speed Insights and 'A' GTMetrix. I would like to know what you guys think I am doing wrong? or what strategy can I work on to get back on track