Never tried Blender, but Gimp has a learning curve that's damn near verticle. Easy to use, it's not.
I use a very old Animation Shop 2, but I don't think it's available anymore. V3 is available as a
30-day trial download here. It should be fully functional, but it'll stop working after a month unless you buy it, or...fix...it.
Pretty simple to use, but not sure if it's what you want...it's frame-by-frame animation. (At least, the old V2 is.)
--A