I don't think it's at all possible to make cross-platform GUIs that feel native. It's of course fine to share the core of your application across platforms, but you have to make the UI part separately for each platform for a truly nice result. There's no escaping that. And it's not like companies like Slack and Discord lack the resources to do so — they absolutely deliberately continue stubbornly ignoring the fact that, setting aside excessive resource usage, no one likes UIs that look and feel out of place in their OS. They totally have the resources necessary to rewrite their apps to use native UI toolkits on all supported systems.