@SuneAuken I've used LO for years, Impress for church services the last few years.

Occasionally have to "import" from PP. Image background slides are pretty straight forward but the complex on-the-fly geometric backgrounds cause Impress problems. My solution: delete the contents, show the slide, screen grab or recreate in something like Gimp.

pptx is useful because you can access the assets after unzipping.