How do I stop stacked die-cut letters from looking lumpy?

The buckling is coming from the glue stick more than anything else. Glue sticks go on uneven and they tack up fast, so by the time you're smoothing the second letter down the first one has allready started to set with wobbles baked in. What fixed it for me was switching to a thin liquid glue applied with an old gift card or a fine-tip bottle, spread in a very light coat edge to edge. Thin is the whole trick. A heavy coat of any wet glue makes the cardstock fibers swell, and that's the rippling you're seeing from the side. I also run each finished letter through my die cut machine between two clean cutting plates with the cutting plate sandwich, no die, just pressure, for about ten seconds. That presses everything dead flat before the glue fully cures and it stays flat after. On the foam squares, yes, they work for stacked words and they actually solve a diferent problem too, because the word gets a shadow and pops off the card. But your instinct is right about thin letters. If the letters are narrow, the foam edge peeks out on curves and tight corners. Two workarounds: trim the foam into small strips with scissors and keep them well inside the letter edges, or use the thin foam tape instead of the thick squares. The thin stuff hides on almost anything wider than a pencil stroke.

For the mushroom and leaf situation, my rule is one focal point, one supporting cluster, done. So the word is the focal point, and then one cluster of elements, say two or three pieces grouped in a corner or tucked along one edge of the word, and you stop. The reason your cards start reading as collage is probably that the elements are scattered evenly around the word. Clustering them in one spot looks intentional. Also try placing a couple of pieces so they overlap the word slightly. Tucking things under or over the focal point ties the whole design together way better than floating everything around it. And your husband can rest easy, the restraint thing genuinely does become instinct after a dozen or so cards. You'll start noticing that the second you want to add 'just one more leaf,' the card is already done.

One thing I learned the hard way with the book trick: put wax paper or parchment under the card first, or the card peels right up with the weight. Also I press each letter down with a bone folder as I go, working from the middle outward, rather than hoping the overnight stack fixes everythign. Catching the wobbles while the glue's still wet saves a lot of grief.

On the clutter thing, my trick is snapping a phone photo before I call it done. The camera flattens everything and busy designs jump out imediately. Half the time I'm peeling a leaf back off after looking at the picture.

Another option for the stacked letters is cutting the same word three times and gluing the layers to each other, then attaching that thick stack to the card as one piece. I do this on most of the cards I sell at the winter market, maybe 60 a year, and it gives you real dimension without any foam edges to hide. Cheap cardstock works fine for the hidden layers, save your good stock for the top. Only catch is you have to line them up perfectly before the glue grabs, so work fast and check the edges.

For the mushroom situation, I do a squint test. Step back a few feet and squint at the card. If your eye lands on the word first, you're good. If it wanders around, pull something off.

Update: the bone folder trick from the middle outward made a bigger difference than the book did, and parchment paper underneath saved me a few near-peels. Word still sits a touch wavy at the last letter but it's way better. I like the idea of cutting the word three times and stacking it as one piece. Does that work with thin script-style letters or do they tear when you handle them?