One time I started working on one open source alternative, but one big problem I found was that their PSD file documentation was incorrect at a few places, and it lags behind what is currently available.

And though Photopea[0] is not open source, it can mostly get things done for my minimal use-cases.

[0] https://www.photopea.com/

Affinity Photo. https://affinity.serif.com/photo/

OK, not open source! but single-purchase at a reasonable price. Which reduces motivation for an OS replacement.

Give a shot at krita it's good for digital raster art. Not for image processing.