Don't give up the rhino tour, it was awesome, and I think it's only offered one day a week! I think a 9am ADR would work. Boma's a buffet, so just monitor your time carefully. After you eat you still need to get to the bus stop, get to AK, into the park, and to the Safari area, and you'll need to check in at least 15 minutes prior to the start of the tour. I'd think leaving Boma by 945 at the very latest still gives you enough time to get to the rhino tour (but if it were me, I'd try for something closer to 830, just so I could relax a bit).