You could indeed put a better quality substrate in areas you want to plant.
I would be concerned that if the 2 materials (sand and plant specific substrate) are different weights one will drift under the other, not stay in place. Using sand and EC might be safer that way.
If you like the look of Safe-T-Sorb, you could use 100% STS, no sand. It is a high CEC material that is really good for plants. I would not use sand in the same tank unless there was a physical barrier between the 2. I did that with Soil Master Select, and the sand (heavier) kept moving, ending up mostly at the bottom of the tank, and the SMS on top, but just mixed enough that they cannot be separated.
If the substrate leaches ammonia that can be useful if you are doing the fishless cycle.
If you will be cycling the tank faster then you would still have to wait it out until the substrate quits producing the ammonia.
I would do the hardscape and planting, then do the fishless cycle.
Safe-T-Sorb and the related materials, sand, EC and many other substrates do not produce ammonia when they are submerged.