I'm liking the composition!
Regarding the missing elements, though... as both xcalibur and Syna stated elsewhere (and long ago too... yikes!) you don't need to stick to the exact same symbolism if it doesn't fit, so long as you can find alternatives that can practically mean the same thing, or emphasize different aspects of the same card.
For instance, you're right that Lucca isn't a flowery person. We also know that now we have two Suns to contend with (the actual sun at the back, and a sun-stone that she's holding). But the Sun card also represents Reason and Math, right? So the first thing that popped into my head was a Sunflower-shaped Sun-like clock or machine at the back she'd engineer with her own hands (and it could also have a face on it; the hands would be the Sun's mustache or something). You can easily kill three-birds with one stone that way, including being a little bit more direct on what the Sun represents, and what Lucca's significance in the story is.
Additionally, later on you can also add small subtle designs on the Sun Stone itself, so that we know it's indeed a Sun Stone. (Maybe a sunflower or sun-rays on it, or something.)
As for the smoke instead of the sun-rays... I'll withhold my opinion on that one, since I think xcalibur's opinion on this would be far better than mine, especially in regards to keeping the movement in the art but without making it appear like it has a Reverse Sun meaning. (Something tells me we could probably depict that movement with the banners at the back themselves).
xcalibur, my dude, where are you?!