It is incredible that the question raised in 2018 and plenty of solutions proposed are not implemented, or maybe they are but this has not been said here.
So what you say is even if I want I cannot say to the app save forecast for this region for next month and go offline? Because this way you would break no agreement.
Then, as you say, it should be necessary to use a special Windy GRIB application and sailors would ask too for an additional routing function.
For shorter passages, a good improvement on Windy should be to offer the possibility to keep in cache the forecast up to 5 days for a limited region. But this is not suitable for longer passages.
(To do that for 2 or 3 days offshore, I use the video screen capture on iPad, but then you need to stop regularly and have a beer in a WiFi spot bar to refresh the forecast... and you too)