Check out Shotr (https://shotr.cc) for mac and flameshot (https://flameshot.org/) for Linux! These apps are totally indispensable in my day-to-day and it sounds like it’ll solve your problem.
Kolourpaint is exactly that for Linux. It's a classical MS Paint clone, and it can take screenshots and crop. Is Qt based but works just as well in other window managers than KDE
Mac has preview.
And some of the Preview features for editing/annotating images and pdfs are now included in Quick Look.
i use mtpaint, which is fantastically old/ugly/tiny and perfect for small jobs