Okk let me dig deeper in this issue it will be fixed by next update i assure you.
I updated the plugin with the new thumbnail generator. Godot does not load all the thumbnails by default so I had to create my own system. So you just have to open the scenes that you want the thumbnails of. It's one time process. Next time you open the editor the thumbnails will always be there. And I fixed few bugs as well related to the scaling issues. Do test it and let me know if there is any bug remained I will fix it by next update.
Cool! Seems to be mostly working, the thumbnails do feel a bit squished though, I think this may be because whatever logic you have to generate the thumbnails is trimming the transparent excess but the preview always expects it to be in a 1:1 aspect ratio.
Either the thumbnails should be generated in a 1:1 aspect ratio, or the TextureRect/Sprite2D (whatever underlying node you're using to display them) should not stretch it to 1:1 at all times)
Also, 2 large chunks of assets got their icon generated quite quickly but now every time I hit refresh it just does the one asset before the last generated thumbnail
If my assets are like this, 50 with no thumbnail, 50 with a thumbnail (in that order), then the next refresh it will be 49 with no thumbnail and 51 with a thumbnail (again in that exact order, instead I would expect it to start from the first asset and work its way forward in chunks)
Oh and also just noticed that the screen scaling got fixed, the sliders and values no longer overlap or get cutoff all over the place!
As for the grouping of assets, could you add a way to remove certain assets from a group? Got a bunch in my favorite group that I added when I was first trying that out but I can't seem to remove them from the group (and having to delete an entire group and then re-add everything that should be in there obviously isn't the way to go)
Edit:
I somehow created 2 Favorites groups.. One has the abbreviated name, the other in full.. They seem to have the same assets and also the original Fav group got cleared somehow..