A bit late but in case you are still looking for a solution.
I have a detailed answer here: Split viewport into areas of different contents
But in short its a plugin. You can use the UMG Splitter(from plugin too) on it, and place the Viewport on the other side, while filling other sides with UI only…
You end user should be able to resize the UI just fine.
Plugin link: https://www.fab.com/listings/b8777f94-8b76-4af9-a110-3c6329b31d06