I have a 75 gallon too. I went with Flourite Black and sand for my Dutch style aquarium. It took about 4 bags of Flourite Black and 2 bags of Flourite Sand.
I have other tanks of various sizes and I have tried the following substrate.
1) For my Fluval EBI shrimp tanks I used ADA Amazonia - by far the best but also the most expensive because of the cross country shipping.
2) I used EcoComplete in my 20L. Growth is good but it is difficult to get any of the stem plants stay down but once the plants take root, they grow just fine.
3) I used Flourite Black in my 29 gallon and used tab roots. Everything grows fine and the stem plants are easier to plant.
4) I used Fluval Stratum in my 10 gallon. By far this is my least favorite. I would not recommend this substrate because it is too light to hold down stem plants, it crushes easily and turns to dust, and it clouds up the water with any movement.
So far I like the Flourite Black because it doesn't cloudy up the water too much when I move plants around and it's easy to plant stem plants. It's available at my LFS and I don't have to replace in the future.
FYI: I did not see any benefit to mixing the Flourite Sand to the Flourite Black.
Hope it helps. Good luck with your project and continue to share your journey