Turns out, that in 2018 about 30-40% of new conversions were coming from the app's mention in Zapier's article about time trackers for Mac. This article was ranking really high in the search results but it got rewritten at the end of 2018 and Qbserve instantly lost a huge chunk of revenue.
Getting back to square one after a whole year of promotion efforts was too demotivating, so I stopped doing marketing and the revenue was slowly falling since then.