I didn't say in advance. After the fact is likely best, but in a seamless flow that doesn't require using Finder, the terminal, or any other tool. Indeed Gnome solves after the fact, allowing you to just accept the default generated filename, or specify your own, with zero friction.
As others in the thread mention, once you've taken the screenshot, it helpfully dumps you in "the editor" (is this Preview.app? I don't know because it doesn't have a title bar..). In the editor, I am give then option to do a bunch of things, but no option to rename the screenshot. This is disappointing. Hence I was hoping somebody here could fill me in on the trick that will make me happy..
(my complaint scoped to Big Sur btw)