They actually say that doing baby voices aren't great for when the kid is developing language skills past infancy. When the baby is past infancy you should talk to them normally and you will be amazed how fast they'll start talking. For example, Northernlion and his wife speak conversationally to their daughter and she's become an incredibly fast speaker, with a large vocab since her papa talks about a lot of subjects. She is 3 years old and can read already.
https://www.youtube.com/watch?v=Z2eRzL1mV2QEven small babies can benefit from the parents speaking in a regular tone to them especially if you leave a few seconds for when the baby is 'supposed' to speak back, this can get them to say their first words faster since they now understand "speak after someone else speaks". Babies are amazing. I want to put a baby in Shu Yamino.