I'm glad you've mentioned that you have experience in both Marketing and Growth Hacking, because if you are planning on going into Consulting, that's mostly what you will be doing when you start, with a bit of sales added in.
If you'd like to be a developer, great, but first get a steady income. Developing software can take a while to get something that will become revenue generating, and creating a full-scale marketing push for something is time consuming. You will be working for yourself unpaid until you have money coming in from that source.
So, find someone to support you while you work on your more lucrative project.
Establishing yourself as a consultant to start will be difficult if no one knows who you are and you don't have the leads to get jobs in your area. If you have a client list already, then you're all set. If you don't, whip out those skills at creating a targeted lead list and get calling and pounding on doors until you find people willing to pay you. As a developer you have options like Guru.com and Freelancer, but you can may make money by going to the local coworking centers and networking mixers in your town to find people who are looking for ongoing developer talent.
Next, figure out what revenue model you want to follow for your own project. Schedule 2 hours or so per day for your own thing in addition to the work you're doing for your day job. Block it out, and then you'll be able to get something done quickly enough.
My recommendation there is to look at the top app lists on all the platforms, see the commonalities and try to find something that is either missing on IOS or can be done better by you. Then, simply do it and use a full scale marketing push to make people aware of it.