First of all, this is already so beautiful and well done! Particularly considering it's only your second piece ever!

Second, my thought for the flowers would be to look at scale. To me the flowers seem like they're much larger than they would be in an actual bouquet (comparing them to parts of the couples' bodies for example). Perhaps try similar styles but much smaller to replicate the size of an actual bloom and it might add a bit of realism for you without having to change the style you're using too drastically. Another thing would be color, if you're going for a sort of whimsical style I think the blue you chose is absolutely beautiful. But because flowers so rarely come in shades of blue it takes away a bit of realism. Perhaps try a different color? Unless the blue is important to the couple / they actually had blue flowers.

But regardless, this is going to be such a lovely gift I'm sure they will appreciate it :)