Thank you to everybody for the critiques on my rough! After many thumbnails I decided this composition seemed to solve some of the problems so I did a few iterations of it.

Some things that you can't see from the thumbnails but that I hope will solve some of the problems:
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Some of the letterboxes are yet to have mail delivered to them (in the foreground)
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Some of the letterboxes have already received deliveries and will have mail visible and the little flag up
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3 of the thumbnails show the mailman actively delivering mail to a letterbox.
The other thing I should explain is that I played with another concept I had — instead of having the glowing object be something at the end of a fishing pole with which to steer the moth, the 2 bottom thumbnails have little "light bulb" creatures on top that light up before delivery and turn off/go to sleep after delivery — the ones in the background will have unlit "bulbs" and the ones in the foreground are glowing with happy little glowing creatures inside with cute, smiley faces.
And finally, there will still be clouds out of which the letterboxes are emerging — I just didn't bother to draw them for the thumbnails.
Sorry this was so long! Thank you for reading, if you're still here. Let me know what you think!