On a day when park opens at 9am, ADRs start at 8am or 8:05am, they start letting people with ADRs into the park then. But your BOG ADR is 8:45am, they will let you into the park earlier, but I don't know if you can get into BOG early? We've only done BOG for lunch, we weren't allowed to check in until 5 minutes before our ADR time.
If you're in your car anyway, I'd be inclined to drive to MK. Driving to MK + monorail might be faster than taking a bus, depends on how long you have to wait or the monorail/bus. Buses start running sometime between 6:30am and 7am, and do take you directly to MK, so that's not a problem. However, if you haven't checked in yet, you won't have a parking pass indicating that you're staying at a Disney resort, you certainly don't want to pay $18 to park at MK when you don't have to.
The other problem is that the official MK opening is 9am, but they've been letting the public in earlier, at around 8:45am. So while you're just starting to order breakfast, park guests are rushing in and doing rides. I know BOG ADRs are difficult to get, but I personally wouldn't want to be eating breakfast during the first hour the park is open when ride lines are shortest.