Chef Mickey’s does not do lunch. Breakfast “ends” at 12:30, but that is just the latest you can make an ADR. We had a 12:15 in 2022 (it was family style then, not buffet), but my daughter and granddaughter were running late and they told us we may not get to see the characters. My daughter finally arrived around 12:35 or so and we were seated and brought lots of breakfast foods! The person in charge of the characters made sure that my granddaughter met each one at least once, even twice! It was nerve wracking trying to decide what to do while waiting for my daughter to arrive (no way did I want to spend that kind of money and not have the character experience!), but it ended up being fantastic! We pretty much had the place to ourselves and they didn’t rush us at all. (Dinner doesn’t start til 5)
The characters do have commitments in other parks, so if the characters are really important to you, I’d commend getting there 10-15 minutes early just to be sure you see them all.