Howdy there Wendell! Nice to see another builder in the KC area.
.124" is indeed on the thick side, especially for an OM, and even more being red spruce. But .080-.085" is on the extreme thin side, and not advisable for a first build since it requires a good feel for stiffness, and different bracing than the standard X with diagonal tone bars... probably somewhere in the .100-.110" vicinity would be good, maybe a bit thinner around the perimeter.
For me, fingers are more useful than ears when it comes to thicknessing the top. Bracing stiffens it up so much, the free plate tap tone doesn't seem to mean much, although I do still tap on it because why not? Flex it with one hand at the tail and one through the soundhole, thumbs reached in as far as possible. Not hard enough to break it of course, but enough to feel how bendy it is. Just go by numbers and guessing at "good" tap tone and flex at first... after a few, you'll start to develop a feel for the range of stiffness we work in.
You don't need a thickness sander for spruce. Hand planes can make quick work of it, followed by a cabinet scraper to smooth out the plane marks. Although depending on whether the wood has much runout, it can be a little tricky to plane without tearing. I've had bad luck with LMI's top wood in that regard. Have to go up on one side of the plate, and down on the other, and it's tricky to get the center level without going across the line and tearing out in the other half. But it helps if you can get your blade really sharp, and take fine shavings with a very small mouth opening on the plane.
Always thickness from the back side. Small tears are much less noticeable inside the box
Good luck, enjoy the journey, and then build another one
