I can't find anywhere how to save my frames as .png sequence with alpha channel. Frames are semitransparent grey. I need it because then I want to convert them into .webp animation with another tool. I tried 'Frames Convert' and set png, but the checkbox for transparency in 'Export' dialog is unchecked and disabled. I understand, that standard video cannot handle transparency, maybe that's why this option is disabled.
The only workaround I found is to save them as non-transparent pngs, then pull them all as layers to GIMP image, then apply 'Color to Alpha' with white to all of the layers and finally use 'Export layers' plugin I have. But this seems dirty and not always applicable to any type of image. I lose frames' grey color and get black instead.
How can I achieve this easier and in more correct way?