Can kasha varnishkes be made ahead?

Yes, and have done it many times.
Washington Post recommends adding half a cup of chicken broth or water to the cooked kasha varnishkes, warming half an hour in a 325F oven, covered with a pot lid or foil,
Other options - microwave, rice cooker or instant pot. Timings per your machine settings, or start slow and check intermittently.
Possible - stovetop in a covered saucepan, again with some liquid added, over medium heat. But this would require more attention and repeated stirring to make sure it doesn't stick or burn.
Enjoy!