I've played around with it a bit - someone packaged Krita as a snap for Ubuntu and I wanted to test installing it. Looked good, more for painting like you say but looked powerful enough for basic image editing. I've seen people produce beautiful digital art with it
Inkscape and Gimp are the stalwarts, but for someone coming from Sketch or Photoshop they feel quite ancient and somewhat messy.
https://www.figma.com is cross-platform (web based), and a pretty good Sketch competitor
For a Lightroom replacement, Darktable is actually quite good (though I really miss the high quality shadows/highlights algorithm from ACR)