Does that mean we need to be on the bus at 6am? We live in the central time zone, so 7am WDW is 6am for us :-S Maybe if we wear the kids out in the pool and drug them with melatonin (kidding...kind of) we'll get to sleep early enough! We typically commando the parks from close to opening to fireworks, but a 6am start may mean an afternoon nap at the hotel for everyone.
I'm hoping someone can chime in about buses for you, but here is some possibly helpful info. We went recently but were on a monorail resort. The monorail started at 6:30 and that is the one we caught for those morning.
With one more stop on the line before MK, we when we got there we were about 3 rows of people back from the left arch, both days we went. The right arch had more people in front.
I had entertained ubering to contemporary and walking over, but the timing was completely fine for the metro.
We did have the buses the time before that, and we were further back at the arches, but it was completely fine; I just don't remember the timing at this point.
Here is what I will say, the rides at 7 were open and fantastic. But, they were also great at 8! i wouldn't stress about getting there later if it's going to be rough due to the time change. Get a fastpass for 7dmt if you want to ride it and then check out the timing when you get there to see if you want to jump on in the morning, too.
If you really want to try hitting the 7am, you could make it a short day, as you said. Enjoy the park however you set up your touring plan from 7-9:30, get a quick bite, then do your fastpasses 10, 11, and noon. Go back to the resort, eat (or eat before you leave the park), nap, pool, and relax that evening, or, if you have enough energy, head back out.
I rely on touringplans. (Sorry if you already know tp and this is old news..) You can see how much you can do if you plug in what you want to do. I created three plans for emh mornings, one for a 7am start, one for 7:30am, and one for 8am. (I always pick relaxed walking to account for bathroom breaks and the random short stop to look at something.) That morning I pick the one closest to what I feel we can do. I use the lines app inside to adjust as we go.
I make the multiple plans as we don't know if we are going to be able to do it. We did hit it both days we went this last time, but the second day was a struggle, as it was near the end of our trip and we were just getting tired, generally. I felt I was possibly going to keel over on the slight uphill walk to 7dmt, heh. And I was yawning during the ride, thinking how I might fall asleep before it was over!