As a developer freelancer as well I would recommend two things, first think of a product or service you can make once and then support and sell, as that way you can scale. Things like support do scale but can also swallow time that you were not planning, and like some of the other comments say can often be a hard sell.
Second with your product or supporting a service as you first suggested go after a vertical market or two and then tie down what they need and automate as much as possible. If you can automate it but sell it as doing the full work you are saving your self time and so profit.
This was some advice I was given for digital products. As an idea a lot of people have is write an ebook on a technical subject, but writing books is hard for the price you get. Start in the middle of this ladder of products and then fill in the steps. As the best way to sell one product is have a companion product that people can choose to spend more or less on.
| ^ |
Custom training/consulting
| ^ |
Seminar
| ^ |
Speaker
| ^ |
Webinar
| ^ |
Video
| V |
Audio
| V |
Book
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So if you wanted to write a book, start with running a 2hr - half day training seminar for 4-10 people. If you can get them to pay a days pay to attend all the better, if not just video yourself to camera, or do it as a free trail and get recommendations from them to run it paid again with other people. Product 1. As talking to people is the most natural and you can just go with the flow and get their feedback its much easier than writing (ok so maybe you write notes first).
With that pay a friend or company to video you, edit and sell this Product 2.
With the video cut off the visuals and check the edit again as an Audio mp3, Product 3.
At each stage you can find the price and demand and see what your customers like, finding new ones or selling back to the old.
With the audio you can pay someone to transcribe it with a cheap paid service or on your own. This will then give you a draft of a book you can work with. All for the work of prep + half day + video edit + audio edit + transcript (much of which you can get someone to do for you) Much less work than a year of writing a book.
You can then choose if customers want to start at top or bottom of the ladder and offer speaking or custom consultation on top of it or using the experience to do a new product and sell back to them.
Be interested how you get on, which ever route you choose. I am trying a SaaS app as a 20% of my time project. Then write and talk about it as I go