I know the official reason for not being able to choose a default layer is cache fragmentation. And the secondary reason is that "this is windy and not radary etc" which likely is more of a joke, but anyway 😛
I'd suggest adding a setting "remember last open layer"
Think of it this way, if a lot of people from a certain geographical area are looking at radar (or any other layer) instead of wind, the caches would actually adapt to that behavior and (could) make the experience better for all.
Label the setting as "experimental" and kill it if it causes trouble. Or make it available to premium users only.
Also, I would not mind if the app still preloaded wind layer data on background, or if the caches were configured or set up in a way that gave preference to the wind layer if you think this is needed to preserve decent initial load speeds for most users.
Two things to balance and I'm sure you've thought of these: initial load speed vs getting to what you want to see speed