I was facing the same problem and was applying multiple work-arounds through blueprint code to get the desired result and that too wasn’t perfect. Finally by fluke I happened to use ‘Movie Render Queue’ plugin for my other project and noticed that UI was not part of the final video. Same plugin I used with my original project and finally without any work-arounds I got the desired result. 
Movie Render Queue is not enabled by default and it has to be enabled before it will start showing (I still use UE 4.27. I don’t know if the plugin is by default enabled in the latest versions). The plugin might seem a little overwhelming to people who are used to the original render sequence feature and will need to follow the documentation to actually learn how to use it. But once you get the hang of it, it is totally worth it.
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