Great effort and reason to create this! I'm accustomed to using the Magic Wand tool in Paint.net[1] and Pinta[2] to select pixels based on color. Any chance you could add that functionality?
The name of this project brought many memories of the HP 48G to the forefront of my mind, after so many years. Its 1 bpp pixelated icons and drawings were indeed called GROBs - there are collections of these online. What a coincidental but fine name for a pixel-capable graphics editor.
Pixelmator - 1000%. Native MacOS - quick to load and very intuitive layer-based paint interface. It's a one-time purchase for $50 but they used to run sales.
They were bought by Apple recently but still sell it. I moved to it when I migrated away from Photoshop. It doesn't have some of the fancier stuff like puppet mesh warp, but for an all-around editor I'd argue its unrivaled in the Mac space.
Very actively developing this project, so hopefully it'll become a suitable replacement relatively soon. Please open GitHub issues or PRs if you have specific requests, happy to help.
[1] https://www.getpaint.net/doc/latest/MagicWand.html
[2] https://www.pinta-project.com/user-guide/wand/