hmm, DYT she's had enough sleep when she wakes early? If so, then it's prob like you say a function of her dropping the nap but perhaps needing less night sleep after a nap day.
At this age though, you might be able to encourage her to wait for the sun if you let her know what she *can* do instead (depends on her temperament though! and I see your ticker says she's spirited..).
We do reward charts but it just seems if she sleeps she sleeps if she wakes up that's it regardless of what incentive is on offer IYKWIM
the incentive (a sticker on a reward chart?) might not be enough, and a bit abstract/detached from the situation, to keep her in bed.
Could you let her know what she can do, quietly, in her bed/bedroom if she wakes up before the sun and can't/doesn't want to go back to sleep or just rest in bed? E.g. could you leave her a few books to look at, some small-world toys...? If you tell her what she can do, until the whole family is ready to get up, then perhaps that would help? You'd prob need to remind her of this a few times but it's worth a try to see if she catches on to it.