Your determination is paying off. It’s a great challenge but there are ways to overcome it. Whether it’s tiles or pencils or other things that are thick, it gets complicated to jig up a solution.
Here’s how I would do it with a full cardboard box as a jig. The box is 1.25" high so the blocks will be proud of the surface. With snapmarks, it would be easy, but the final alignment of the jig for repeating the operations and then flipping the blocks is a challenge, but there are ways to get the file aligned correctly. Your pizza box standoff idea is right on the money. Once you have the jig made and the blocks in, you could always just do one test block and align that. The rest will follow. That’s why I have one extra outline of the letter W in the center that you can score and align properly.
Using cardboard that is .15" thick, you could make the box for the jig, and fix it well to the center of the usable bed area. The material thickness of this would be 0.04" for its surface.
Then put the blocks in and you would have the surface height of the engrave for the top of the blocks in the jig .298".
At least I hope this would work. I’ve been wanting to do blocks some day and I figured I may as well take this opportunity to try it out. The file has the rest of the box jig outside of the viewable area. They are different colors so you can do three operations to get the whole box printed.
And are the blocks 1.51" or 1.5" exactly? I didn’t notice the dimension in the title, just remembered 1.5. I made the cutouts 1.5" with no kerf allowance.
Final question: For all the folks who can think algorithmically: for doing the other sides of the blocks, would you shift each block six spaces down the series for each side? What is optimal for dispersing the letters and numbers out?