If you are lucky with spawns. If unlucky a balloon may not spawn for 30 minutes. I usually did it while watch tv or something. Not exactly fun, but if you can multitask it isn't too much.
You shouldn't be getting fake art if you verified they are real using sources. There is a 10% chance that Redd will always have fake art. Maybe you are getting that 10% chance and thinking it is real?
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I would say color variations don't count because there is little work involved with changing the color of an 3D object.
For comparison New Leaf had 1764 (unique) furniture pieces, while New Horizons only has 1131.
Because the Switch should be able to handle more. The design (and storage) limit seems arbitrary especially in a game centered around creating your own island.