Right. If you insist on the granite, then support the miter slot by epoxying on a 3/4" thick piece of granite.
Some thoughts about this. It seems to me that granite brings more negatives to this than positives. How flat and how thick is this stuff? What kind of infrastructure do you plan under this thing? The granite countertops I've seen are fully supported by a substrate. They are susceptible to fracturing under relatively slight impacts. There is no need for them to be dead flat in this application. And weight can become an issue as well.
The only thing a router table MUST be is flat and REMAIN flat. The reason is that the table top is a reference surface relative to the bit. The best use for a router table is for routing pieces that are too small to support the base of the router when routing. However, the reality is that most use the router table and router as a shaper. Therefor choose a top material that is flat and support it underneath so it remains that way.
With regard to miter slots, consider this. A round cutter like a router bit has no need to pass the material parallel to it because there is no parallel with a circle. A saw blade is a linear cutter and material has to be presented to it at one angle only. A miter slot parallel with the blade is useful, even necessary. Not true with a round cutter.
One more thought, just because there are large router tables, miter slots in router tables, and drop in router bases, doesn't mean that those are the best idea. The thinking we bring to the router and its use is too often lock step with linear cutters like saws. And that can complicate the use of the router and make things unnecessarily difficult, even unsafe, and hugely frustrating.
Just a couple of centavos from the Router Lady (retired).
For more info, come to Burning Wood at Brent's in Nevada in June. Or track down a copy of my book, Router Joinery Workshop. It was printed 10 years ago but the principles of operation don't change.