Yes, the best next step would be registering a trademark.
A domain name secures your address on the internet, but it does not give you exclusive rights to the brand name in commerce. A trademark gives you the legal grounds to prevent others from using the same or a confusingly similar name in connection with the same goods or services, to oppose similar applications, and to take action against infringers. Without a trademark, another business can trade under a similar name (even one you have established), and you will have limited legal recourse.
In other words, owning the domain is a good start, but if you want to protect the brand behind it, you should be registering a trademark.