Thanks to Martin Wimpress (@flexiondotorg) from Snapcraft team, our snap target is greatly improved. Downside — size increased from 75MB to 108MB. But we consider that it is worth to (and in the future size of snap will come down).
Also, as part of rewriting some part of electron-builder in Go (that much more suitable for command line tools compared to NodeJS), graphicsmagick is not required anymore on Linux and Windows (on macOS bundled sips was used previously, but now also streamlined to one unified implementation).