I'm happy to close this PR unmerged if we could find anyone else volunteering to do the good way.
If it wasn't that #3209 is a crash I'd say leave the windows dialogs until GTK4 forces them out, after all they have been used for a long time without issues.
But since its a crash I'd say merge it, if any white knight rides in on their trusty steed I suspect the olde code would not be much help since the Windows dialog APIs have changed, but they can always choose to start with the commit before merge of after, their choice.
Or perhaps someone will try https://docs.gtk.org/gtk3/class.FileChooserNative.html and see if it works better on windows/macos and flatpack (see docs).