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.
I love you. And the person who made Shottr.
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
i use mtpaint, which is fantastically old/ugly/tiny and perfect for small jobs
I advise you to try Paint.NET, it’s a free and modern reincarnation of Paint Shop Pro, which was a true killer.