I'm making about 200 USD/month by selling watermark removals on https://gifmemes.io. What I've learnt is that I'm very bad at marketing & design and will much rather spend hours programming useless features. For example I've spent like 10 hours getting an wasm ffmpeg converter of gifs to mp4s and webms, which was not used AT ALL, while most of my traffic still comes from cold-writing (is it even a term) for 1 hour random people to put links on their websites or subreddits.
The scene in the Sillicon Valley series, where Richards enhances the algorithm by 1 % is quite accurate.
On the other hand, being the solo developer and product owner is quite powerful. My implementation of billing into the app took around 2 hours, which is probably less than a meeting about it would take in a company.
Finally, I can run silly experiments. There is a button in the "Buy watermark removal" page that says "It's too expensive", which leads to 50 % discount. Around 70 % people still buy the full price.