If you're looking at drawing ships and trees as a business venture? are you making a profit from this line of business? If so, you should definitely niche down and get really good at it, find new techniques, and ways to make it the absolute best product to solve the needs of your customers. If you're doing it for fun, feel free to draw different things.
These are great by the way.
Thank you very much.
I have been making art since 2017. And only have sold 5 pieces:
As you can see each art piece I sold is different so I don't really have a single niche that sells well.