"Since the filament is going to expand - do you really want the middle line?" : Yes since the design of the part relies on this characteristic; So i need to fill the gap here for this part. Ortherwise i have an inside wall and an outside wall and no connection between them and the top is a mess, look at the picture. This is an RC plane cone which needs some resistance.
Cura 4.x handles this with no problem. The way Cura 5 changes its algorythm for this is strange ...
Concerning the flow it mus be set lower then 60% at max, otherwise the filament will not give the expected result.