There are a few jobs or businesses that people traditionally imagine will be easy to set up. Here in the UK moving to the coast or to a beautiful rural area and running a small guesthouse is often suggested as a perfect work/life balance. The idea being that you serve breakfast and then clean a few bedrooms and your workday is over by 10 am, and you have the rest of your time is spent on the beach or walking through fantastic scenery. The reality is that you are actually on call 24/7 and if someone manages to block a toilet at 2 am, it is your job to fix it.
Another common small business idea is to work farmer’s markets or local produce markets running a cake stall. If you have a kitchen and can read a recipe, you should be able to bake cakes and sell them. The downfall is that every hour spend making cakes is an hour you cannot be selling cakes, and the cakes have to be fresh, so you flog yourself to death baking in the days before a market, and anything not sold on the day is lost.
In our industry, Adventure Writing is like this. Anyone can write an adventure. We all know how to do it. Everything we need is in the rulebooks, normally with advice on how to create a good one.
Not all adventures are born equal. An Adventure Path is a mammoth undertaking and beyond most indie creatives. Not only would it take the best part of a year to write, then you have a huge amount of art to create, then you need the testing and editing. If this is your first gig, you have no guarantee that you will sell any copies at all.
The other extreme is the one-page adventure. Probably just a few hours of work, but these command low prices, which means lower earnings. Another consideration is that the one-page adventure niche is pretty full of well-established names, for the biggest systems (and audiences).
If you want to get into adventure writing, what you need is a unique selling point. What will make your adventures stand out? What can people get from you, that they cannot get elsewhere?
I cannot answer that for you. What you are looking for is that something special, but in a format that is within your abilities and means to create.