Krita is definitely awesome, 100% free open source and much easier to use than GIMP. It is more tailored towards painting than digital editing, but it can do both.
Also not mentioned is that there is an Android version, which also works on ChromeOS. But if you are on an x86 Chromebook, you are better off using the linux version as the Android version is missing a few features like plugins and exporting/importing video via ffmpeg. You can still export/import frames if you use an external ffmpeg manually.
For linux though, I'd suggest the Appimage version over the flatpak version. First of all, appimage is packaged faster(day 1), second of all, unless you have other flatpak programs, the flatpak version will take up much more space. That is because flatpak makes an isolated environment and needs a backup copy of every underlying library that your OS may already have. It's less of a problem if you have multiple flatpak programs that share these libraries, but for a single program it just uses extra space.