I'm a fan of OpenSCAD as you likely know, but I suspect the OP might be a bit of a beginner to the 3D modeling world.

Regarding the letter scaling: take the letter A for example. The center of the A is a triangle. The depression cut from the primary block is going to be a specific size. The letter that created the depression will be the same size. Scale it down and the triangle cut-out in the letter becomes smaller than the non-depressed area in the main block, yes? Effectively the same thing for any letter with an enclosed area, such as P, R, O, etc.

After creating my "instructable" above, I realize that it will not work for this reason.

I'd probably create one set of letters in Inkscape for making the depressions, then inset the paths an appropriate distance without rescaling the letters. This would keep the cut-outs in the proper position and provide necessary clearance. Extruding the paths can then be done in Tinkercad or in OpenSCAD as desired.