OK, thanks @TwistedTwigleg , I think I have it now:
Your Bootsplash image is what will be displayed during the actual boot of the app, while the Storyboard Launch Image will be displayed up front (and only be there for a reaaaaally short blink on most modern iOS devices).
By default, Godot packs the Boot splash image into your Storyboard. Which is why, by default, it will show twice if you check the Storyboard option to true.
You then need to give the Storyboard a @2x and/or @3x image; this is the image that will be shown first, followed by the boot splash.
So, simple rule of thumb for anyone who tries to figure this out:
1. Launch is just for launch: The Launch image (if Storyboard Launch Image is chosen in iOS Export options) will show first, during app launch, and only for a very short moment after the app has run once. It will be the same as the boot splash image unless you set it to a custom one in the fields underneath.
2. Boot is during booting: The Boot Splash image will show in any case after the Storyboard Launch Image showed. No choices there. It will show longer than the launch image.
TIA