Maybe try Spectacle. I use the OOTB Spectacle app on Fedora KDE. It has the same features as Flameshot and is .. well, native.

But on my Mac, I use indeed Flameshot, it's not ideal (the window is "shrinking" when a screenshot is captured), but it's better than any alternative I tried.

Huh? I'm running kde plasma on wayland and flameshot runs like a dream come true. I hit the flameshot icon in the tray, it automatically selects the whole screen to save, or if I click it starts cropping wherever I move the mouse. It's like the devs read my mind for exactly what I wanted