Safe career don't exist. If you work on your own you always take some chances.
Concerning a
SEO career the money you can make depends on your skills and how many companies hire you.
SEO is a field that requires constant attention to any change because search engines and the issues around them evolve with dramatic speed so not only you must work hard to acquire the skills you need to be successful but you also need to work hard to keep up to date.
I don't have a lot to adice about selling your services because it depends a lot on your environment: unless you have an established reputation clients will want to see you in person so you'll have to check how many companies with Internet sites in need of
SEO services there are in your region and start offering them your services. You have also to understand how much they're willing to pay for your services.