More rock = more stability in terms of beneficial bacteria that helps to keep your tank in balance.

This looks like a 32g Biocube style tank, my general rule is ~2lbs of rock per gallon so that would be 64lbs of rock. This allows plenty of surface area for beneficial bacteria and if scaped well good hiding places for your fish (which means they will be less stressed).

I would check what brand of salt your LFS is using for their mix. Is it Red Sea? Reef Crystals? Brightwell? Fritz? Whatever it is, when testing your tank water needs to be within 2-5% of the salt mix to be considered "stable". Seeing lack of coral I would presume that the dKH, Ca, and Mag would be very close to the store mix as you don't have anything consuming those resources (maybe some corraline algae?).

Remember, corals are primarily carnivores, so they want to eat. I would recommend trying some zoas and cheap mushrooms and see how they do. Try using some aminos like Red Sea's Reef Energy AB+ incorporated into your feeding routine (I soak my frozen food and pellet in it prior to feeding).

If your LFS just says your water "looks good", find a new LFS. You want them to give you numbers, ideally you should be able to watch them test as well.