What that soil needs is organic matter, not more of the mineral part of soil, so you do not need "topsoil". Any "topsoil" you buy might contain as much aqs 5 percent organic matter and 95 percent of the mineral component all soils contain, either sand, silt, or clay, stuff you do not need to spend money on. Look for compost, leaf mold, somehting that is organic matter to work into that soil.

Dig down, carefully, next to the trunk until you find the flare, where the trunk and the roots divide, and the trunk thickens outward. That should be at ground level. Using that as your guide, start removing the dirt. You don't have to remove ALL the dirt over the roots, but try to leave no more than 2". If the tree's roots have already started growing up into the new dirt, carefully remove it - they won't be much problem. If they are heavily into the dirt you are removing, then leave it. You may end up with a lawn that is slightly uneven - if you really can't stand that, then apply a light mulch to the area between the "hilly" part and the trunk - no mulch in the 2-4" next to the trunk, and no more than 4" of new stuff (dirt and mulch) over the tree roots. Mulching anyway, as above, and as wide as possible, is a good idea. Good luck.

I like the symmetry of the pots against the building and I would leave that as is.. I group my pots generally in odd numbers and stagger the pot sizes in the grouping, and also mix up the pot shapes in a grouping. My pots look similar, but I pair round with square, etc. I anchor my groupings with a large pot, and I find large pots easier to keep watered. If this were my terrace I would plant some trailing plants along the edge in some pots for a more lush look. I would add a few more pots with some colorful annuals. As czarina suggested, I stack some of my planters. Some of them are stacked on upside down pots. Here we have plant stands for hanging plants - you might like that look with lush trailing annuals. You have a lovely terrace and the plants you have are a nice foundation. My biggest problem with a large pot garden is keeping it looking nice over the winter. I mulch my pots nicely and plant violas and pansies as much as I can for winter. I don't know if you will have that problem in your zone or not.

No roots - no water. Watering is best to start when plants start growing some roots. That may take 2-3 weeks. Some ppl will mist, but not immediately. Your soil seems to be full of peat, at least that is how it looks to me. It will be problem eventually, it may became hydrophobic and that is no good for succulents. You should add much more of perlite or pumice, making mix in 1:1 ratio. It is lots of different plants together. I often root quite a few in same pot, but usually same kind of plants - may not be much problem...