I use gimp to edit (clean up) my scanned watercolor paintings. Yes, gimp is good enough now for what I used to do with photoshop: adjustment layers, more sane ui. Only thing that was missing is a very obscure feature that photoshop has, to merge multiple scanned pages of a very large photo. I now use vuescan for that (the free version does not add a watermark when using that particular feature, unlike its scans!). And then I edit in gimp, or RapidRAW (a new, lightroom-like app, that's easier to use than darktable). So I'm set.

This is how I do it:

• Scan with the official EpsonScan2 app form flatpak as TIFF (unfortunately their .deb file coredumps on Linux Mint). The XSane app unfortunately is too buggy.
• Then I merge the various scans to a single scan (if my painting was too large and needed several passes), with the free version of VueScan. There is one other foss app that can do that, but it's so convoluted that it's not even funny. Vuescan does it with a single click and it doesn't add a watermark, curiously enough!
• Then I edit either in Gimp to fix the wrong scanned colors (this epson scanner moves oranges to red a bit), or fix mistakes (that's common now even for traditional illustrators). If it's only colors I need to fix and not change actual parts of the painting, I might just use RapidRAW.
• Then I export at 1024px high for web usage, as a jpg 90% quality. I then archive the TIFFs and XCF files.