Start new legacy?

My suggestion would be to either wait until infants, or have your founding sims hold off on having babies until infants come out. It's only 2 1/2 weeks now until the update is live, so you could also spend some time renovating community lots for your new legacy's save or something until then!

That's what I would do, anyways. I recently started Gen 6 of my legacy, and I'm holding off on my heir having children until she's worked on her career a bit first, focusing on just that for her and building in the meantime~