Are the higher dimensional beings doing crop circles on Earth actually the Reptilians in the future extension of current reality? Or, do destroyed civilizations of the Sirians and other starseeds, for example, have sympathetic beings still existing on future timeline extensions?

You figured things out nicely, here, that the crop circles are indeed made by the Reptilians on the future life extension timeline. This is why they have been given permission to at least do some things to help awaken humanity and warn them about the evil in their midst, because they have skin in the game, so to speak, and are intertwined karmically with all that has happened and is going to happen on your planet. The civilizations of other starseeds have already had their future life extensions peter out from the consequences of the decimation of their worlds by the Extraterrestrial Alliance acting as predators to conquer and destroy them. So, while crop circles have been attributed, falsely, to beings from many varied star systems, that is either faulty sensing of the origin or disinformation deliberately designed to promote the idea that there are many benign and advanced sympathetic extraterrestrial civilizations who care about the Earth and want to help, when the reality is, it is only the more awakened members of the Reptilian race who are advanced enough to know they are in big trouble unless their forebears are reined in, and stop the evil.