Nice work HS, don't worry, the pics are perfectly appropriate.
I'm thinking that if you made it so the back of the bikini went up higher, like by 1 1/2 inches (or more, and you'd just wear it so the front went higher to match), that could help. Then the bikini would be able to be tied higher and tighter over the top curve of your rear, and would have something to hold it up (your butt! ).
I'm also wondering if the ability to tighten to top edge is being restricted by how tight you can have it around your thighs? You know, if the tension along the "V" lines of the bikini is tighter than around the hips? Then you might need to tweak the construction a bit. Like loosening it around the thighs, or tightening it along the top edge. You might try weaving the chain cord in through the top edge, so it can gather in a bit, as it needs to. That will change the V-angle to be a tad more vertical, and consequently looser as it needs to be, to allow for the top to be pulled tighter. Hope that makes sense. In other words, you should be able to pull the top edge as tight as you need to, without being restricted by the bikini's construction.
Also, if the stretchiness of the chain cord is a problem, or if it's digging in, you could use a ribbon, or other substitute.
Let us know how it goes!