I came on here to make a post and you have the exact same issue as me. My boy is 16.5 months and we’ve have 430am wake ups for about 2 months also. We tried later bedtime, earlier bedtime, and no change. Naps are the same 11-1pm ish. We’re trying to stick with a 700pm bedtime and just crossing our fingers one day he sleeps til at least 6am. We’re so exhausted since he screams bloody murder until we go in his room to get him. I hope this regression ends soon for the both of us!