Hi! I wonder it is 2020 and linux still does not have a stable application with the usability of PaintShop from 1992. Now everybody will jump at my jugular screaming GIMP! GIMP! But GIMP is horrible, I can't do the simplest thing like highligting something with an elipse or underline. It makes me feel stupid. I might be stupid, but every time I want to do something I end up googling and reading tutotial pages and how-to guides and accomplishing nothing until I copy the image in a usb stick and bring it to my XP machine and edit it in PaintShop 5 from 1999 in 30 seconds and I'm done. I have used Pinta but it is very limited in capabilities. The lack of a simple and usable image editing tool is the only thing preventing me to fully transition to linux. If the only thing in the future for this is GIMP, I'm giving up linux after 15 years of trying. Did I mention that I HATE GIMP?
Have you looked at gpaint (a GNU Paint program) or Mypaint ?
In my experience both KolourPaint and GPaint are even more limited than Pinta (they don't even have layer support), and Mypaint is broken in 20.04 (missing icons and crashes).
Personally I am fine with Pinta, but I agree it could use some beefing up.
You may be able to do what you want using "ksnip" (I run it as an Appimage) and "LibreOffice Draw", in both cases by importing the image you want to work on and then applying your "modifications". LibreOffice Draw can work with layers. Hope that helps you.
Or, you could always try a platform independent free online graphics editor