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You can do this with Preview → Annotations

You can have the annotations palette always displayed. I haven't used MSPaint since the Windows XP days, but I don't think it can be any easier than this.

Preview, despite its name, does this really well. Its freeform drawing tools are a little different (shapes and lines are vector, not pixel) but they're very easy to use.

I mostly need to do that kind of thing on screenshots. The Flameshot app has worked really well for taking screenshots and annotating them.

https://flameshot.org/

I would take a screenshot of part of my screen (CMND+Shift+4), mark up the screenshot with a red box then send.

A screenshot is plenty high res enough already. If anything you’ll want to shrink or before sending.

I like photopea nowadays.