I built https://bankstatement2csv.com It's a pdf bank statement to csv converter (which seems like an increasingly popular niche for solo devs), making $550/month as of today.

Marketing is challenging, and have only really had some minor success using ppc. Also, I have a fairly high churn rate (like 30%). From talking with users, it's mostly from individuals or small business owners that convert their statements from the year and then are done. Book keepers and CPAs tend to keep their subscriptions, which makes sense.

Tech stack: Java, Javalin, Jooq, PdfBox, JavaScript, React, Tailwind

Hosted on DO