A bunch of pieces of concrete will always be a bunch of pieces of concrete, any jointing compound that's weaker than the reinforcing bars in the concrete is at best temporary.
Pavers can hide a multitude of sins.
Use a mounting block with a built-in box
The simplest solution to this problem is to use a siding mounting block with a builtin pancake box, such as an Arlington 8141 (the correct part number depends on the lap depth of your siding, the 8141 is what's used with a 1/2" lap) or equivalent.
Your profile says Seattle. The weathering of the wood says that you get a fair amount of water.
That slab is too high relative to your siding and probably the threshold of your door. Modern code says 4" from top of slab to top of impermeable layer.
If you are keeping the house for more than 10 years, I'd ditch the slab, set the grade to the correct ...