As a full time software developer I would find number 4 the easiest of the suggested choices, as its the only one I have done before. (Finding the niche is the tricky part I guess).
But it's definitely a paradox of choice. There are seemingly infinite problem and infinite solutions...so which do you invest your time in pursuing.
actually 5 is a good bet as well for a full time developer. e.g. make a WordPress plugin, give a demo but functional version out for free on WordPress.org, then sell the full featured version on a marketplace like Envato. If your plugin is genuinely useful to someone they will buy it. it won't be enough money to quit your job but it will be something on the side.
7 is arguably easier than 4 from a business and marketing standpoint.
true, the app stores take care of those parts for you. in both cases though it's just a very small minority that explodes into popularity.