[dharma1]

I'm not sure what toolkit Adobe use but I have a feeling it's not easily portable to Linux.

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)

[wlesieutre]

Check out Krita sometime as a Gimp alternative. It's geared toward digital painting a bit more, but it works for general image editing too.

[dharma1]

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