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You can implement the frontend separately for each platform, which is used by example by usbimager, Transmission or SciTE.

This is very hard to do in practice for any big enough software, but then using Qt, Java or C# frameworks is probably worth it since their impact will start to be small compared to your application footprint.

Do like everybody else does and wrap it in a web-browser.

Does wine have the ability to package windows binaries? That would probably be lighter weight, assuming it performed a tree shaking pass.

It is the first time I heard that Qt was bloated. Is this true? If so, I would like to know what other UI toolkits do that is less bloated.

Put UI behind PAL like sumatra does with pdf engine.

Java is still viable for many usecases.