Hi Michael,
I think you need not give up on the basis of file size: remember that your geometry consists of a number of separate letters. You can handle each letter as a separate file.
In the case of the substrate, the engrave mop can simply be replicated for each letter and then the 'generate G-Code' done separately for each.
In the case of the inlay, it is a bit more tricky because the letters are close enough together to interfere with one another. You will have to move the letters apart from one another and generate the inlay for each separately. They will then have to be diced up on a table saw and glued into place separately. This could be a good plan anyway.
Alternatively, you could fall back to doing a conventional marquetry-style inlay. You will be stuck with rounded corners with the radius of your finishing cutter, but that may not be too bad. There is a plugin at the plugins site called "Inlay Toolpath" that calculates the correct shape for the inlay piece, rounding off the corners correctly. I have not used it.
Bob