I say the sooner, the better. Yes, first grade is very critical for reading skills. But is your charter school inferior to the public school in teaching reading skills?
We just moved last year, my daughter was starting second grade. After attending 2 years in the same school, the move was a hard one for her. She had cemented so many friendships in K and 1st that the transition was difficult socially for her. Maybe it is different for boys. After almost the full school year, she still complains about not being at her old school. Good luck with your decision.