I am not sure about any licensing restrictions of generated images, but stable diffusion as such is open source, as far as I know. That playground just charges for it's service to run the software on their servers and might not even be able to hinder you from using the generated images as you please. That will be done by various platforms where you would want to release them. Some will ban such AI generated things outright, because of the uncertain future legalities and currently disputed status in regard of copyright of the training material.
That being said, do you know this project here
It is stable diffusion plus gui to run on your own machine.
Since you are an experienced coder you might have more fine control to adjust the AI to your needs. But I did not really check out the playground, so that one might be good enough or have additional bells and whistles and qol features.