> 2.1) I want to ship something that looks the same on Windows / Linux / MacOS
As a user: No no no, please, no.
I want apps to fit the look, feel, and idioms of the OS I'm currently in, not "look the same" on every OS.
For example. macOS in particular has many builtin features that work in all apps with standard controls. Like text-to-speech, or converting selected text into all-caps, lowercase, or capitalized. Or custom services/scripts built in Automator.
I may not always use these little tricks all the time, but when they fail to work in some app, it immediately feels lower quality, suspicious, and/or suggests a lazy developer.
It's not pleasant to use something that ignores the special features of a platform in favor of the lowest common denominator.