At my last company, we added to our employment contract a place for new employees to list all the outside projects that they had at that time, including open so

At my last company, we added to our employment contract a place for new employees to list all the outside projects that they had at that time, including open source projects they were involved in. During employment, they could add to this list, depending on if the project actually competed with us or not. This makes it clear where the lines are, so there's no ambiguity. You may want to see if you can have this discussion before going any further with your new project.