I develop and sell a Q&A platform since 2017 (https://scoold.com). It started off as Stack Overflow clone but now it has features which are quite niche and missing from SO like geolocation and spaces for teams. The project is open source but has a Pro version which generates around 4000€ per month. I spend around 30% of my work day on support and emails. The rest of the day I fix issues and develop new features. I'm a solo developer and work full time on the project. Accounting & legal is handled by an external company. I operate as a limited company within the EU.