I have always been installing the "legacy" and fairly heavyweight app called Shutter (which needed some tweaking to get annotations to work) but now there is Ksnip which has all the functions I specifically needed such as annotations, timed capture, etc. Bonus is it works on not only Linux but also Windows and macOS.
See their Github page at https://github.com/damirporobic/ksnip and I’ve attached an example screenshot that I took of it using a second instance of the app.
