Now the final render time is much less than I wrote for all the nebulae that have already published here. The render time I was able to reduce almost 10 times, and with this improve the quality of the picture (almost fully removed the fine noise). That I rendered 60 hours, now I can render for 8 hours on the same computer. So this is not the worst thing. You just need to leave the computer turned on for the night and the picture is ready in the morning.
Much worse is how the settings are configured when the entire node network is connected and you need to see the result of a small adjustment of one parameter. As a result, you have to switch to an outlined node scheme or create test scenes and study the result after a few hours of rendering. Or you have to choose a small area (crop) and focus on it, but there is a possibility that the same parameter can have unpredictable influence on other sites that you have not looked at. Or it is simply good to know what and how it works and to foresee the result. The main thing for me now is precisely this.
As for the quality of the render on the free version. Is it different from the paid version? Judging by what is listed on this page
http://planetside.co.uk/terragen-product-comparison/, the free version contains the same render-engine as the paid versions, and the differences are only in separate functional blocks, which are usually required for solid professional work with integration with other applications.