I do not think that it should be a secret that Thingol and Melian have a daughter, nor are we going to 'save' her onscreen for the meeting with Beren. If nothing else, we have to show Daeron being hopelessly in love with her prior to Beren showing up. But also, we are going to show Galadriel becoming friends with Melian and meeting Celeborn, so Luthien would be very conspicuous by her absence. Obviously, if we are just showing her as a baby/small child here, we don't need her adult actress yet this season. She can be a background character/bit part until Season 5, but...not a secret.
The 'story' in her birth is not much of one, I agree - it's that the Sindar have settled down to make a home for themselves in Doriath. In this season, we are pre-Menegroth and pre-Girdle of Melian, so showing them starting families gives a bit more of the 'settled' feel, to show that they've given up the idea of ever going to Valinor. We don't have to do this, of course, and I'm not sure we need to make the point that a Maia and an Elf can have a baby together, but it is interesting, and it is happening about that time, and....that episode still has no storyline, just a theme. I feel we could do with a parallel to the birth of Fëanor story in Valinor, and birth of Lúthien in Doriath seemed the obvious choice. But it could also be the birth of Celeborn, since he is in our Frame and we will need that to tie this episode together no matter what we do.