If it were my child, I’d probably start 1b mid-year (unless I felt the child needed more time). For K, I keep school activities optional. If they want to play, I let them play. If they are begging for school, we do school. For reading lessons, I follow their lead. My DC have showed readiness anywhere from 4 to 6 years of age.
K here is mostly learning letter sounds, free play with math manipulatives (counters, pattern blocks, etc), puzzles/games, and picture books. Also read alouds and projects with the bigger kids as they are interested.