I would NOT take a taxi to an early ADR at Animal Kingdom. The parking lot at Animal Kingdom opens later than most other parks, and you may be sitting at the toll booth in a taxi not able to get there in time.
Disney buses go in a different entrance and are allowed in earlier.
ASMu and AK are not very far from each other, and also that early in the morning, you are seated pretty quickly once you arrive, so, I still stand by my 7:30 bus time. HOWEVER, if you want to eat earlier than your ADR time, get there earlier, you will be seated earlier. We did this once. We had an 8:35am ADR, but wanted to be able to get to Everest right at rope drop, so we showed up at the AK gates at 7:45. We were sitting down and getting our drinks served to us by 8:00 am. We were finished breakfast by 8:45 am and were on Everest by 9:02 am