Hey, just checking how this setup looks for you with images in motion, because I originally did exactly the same setup with the room and camera at 64x64, but with the viewport at 512x512 and it actually didn't result in a true 64x64 effect.
For example, at the start of my game I have an image scaled x2, and I "Zoom out" by reducing the scale to 1. The zoom out was completely smooth, without the visual distortion you get when you're working with such a small number of pixels. It was as though the image was much higher quality but drawn to look pixelated.
Then I changed the viewport to 64x64 and used "window_set_size()" to set it at 512 x 512 and it resulted in a larger window with a true 64 x 64 screen. Not sure whether this is some strange quirk from how I've done other things in the game, but that ended up making it look a bit more authentic!