Guess I'll be the voice of dissent here. We don't mind taking the Disney buses most of the time. We occasionally will have a longer-than-average wait, but we typically only drive to DTD and use the park buses the rest of the time. Once while I was there by myself with my kids, I did drive to MK one afternoon to stay through Wishes, and afterwards I said I'd never do that again. We had to wait for a ferry to get back to the TTC (MK parking lot), then waited through the line for several trams to leave, full, before we could get on, then almost at the end of the route to get to our car and in the meantime, my youngest had fallen asleep so I had to do everything while holding him. Never again. I'd rather wait for a bus.
Re: your specific situation, I would re-schedule that ADR. If you're dead-set on having that T-Rex ADR mid-day, you can drive to MK, then ferry/monorail back to the parking lot, then drive to DTD, then reverse, or you could walk to the CR and get a bus there to go to DTD. That's probably what I'd do. Either option is going to take 40-60 min, from the time you walk out of the gate to the time you arrive at T-Rex. I personally wouldn't take 4 hours from the middle of my MK touring day to eat at T-Rex, but I wasn't that impressed with it.