Thank you for the kind words! You will be happy to know that in the next stable release, you will be able to import gifs without needing to rely on FFMPEG! The importing process will unfortunately be a bit slow for large gifs though because it is entirely written in GDScript, but in the future we could use C++ instead for native performance.
As for the thumbnails, there is not much we can do about that, it is up to each file manager to implement them. There is a thumbnailer for Dolphin that I made which Variable linked, but besides that I'm not sure if there is anything available at the moment.