I like the plan. Having a mound means the water will flow downhill and away from the base of the plants.
Using the pavers as a ring around the base (or focused around the downhill side if this will be on a slope) will give the mound some longevity, though it will settle gradually over time.
A saguaro will become gigantic one day (maybe not in your lifetime) and will eventually dominate a certain amount of space. The roots are pretty hard core, to support all that weight. An ocotillo is also a really large plant at maturity, be sure to give enough space. Plant those closer to the center of the mound but prepare for many years' wait before they reach their potential.
As far as amendments, I would see this as a real opportunity to break up the soil. A combination of fine gravel and organic matter mixed into the soil would improve drainage, especially in the top 2 feet or so. Maybe 10% of the total volume each, as a starting point? You could get more aggressive than that, I think.
Avoid fine sand, any sort of fine material. Avoid vermiculite and perlite. Maybe try pumice if you're willing to spend the bucks. Volcanic rock in general is really nice and porous. River sand for construction is good if you screen it first to remove the fines. Finer gravel for construction is also good, but not the chunky kind.
The organic matter is going to break down over time, and you can expect to lose a significant volume of the mound to that process (so make it bigger than you want it to eventually be). Maybe go for something like composted bark if you can find it, something long lived, neither fine in texture (overly old or broken down) nor extra chunky (mulch size).
To be fair, any time you dig up a huge pile of dirt and put it somewhere, it will subside quite a bit in the first few months. After the first couple of heavy rains, you may have to do some readjusting to compensate for this process. Definitely do not plant anything right away.
Important to avoid stratification (distinct layers of materials) so spend a little extra time to get them properly mixed. Maybe gently water down the bottom half or so before you do the top half, this will help get things to go together. The heat where you are is going to make that water go away pretty fast.
Plan for something to grow along the sides of the mound to avoid erosion. There are various options in this regard, but I don't know what would be best in your climate. Avoid planting any agave that offsets (produces pups) near the edge of the mound, because those pups will start appearing much lower down than you expected. It might sound like an interesting look, but it's kind of a nightmare after a few years have elapsed.
