Worry about the rail now. The posts should be installed inside the frame. Place the footings 16'' square and two feet down or on solid,not backfill, soil.

Some areas require much deeper piers,42/48'' , because of frost heave. J.

I'd second John's suggesting of planning for that rail now. You want to make sure you can bolt those posts solidly to the framing.

I'm a DIYer, not a pro, but I had the experience of building a deck for myself (link below). It is similar to how your deck is long and not very high off the ground. Lowes used a program to generate my first plans. They were very similar to what you drew. I ended up making two significant changes to the plans; (1) many more footing and a second beam, (2) attached joist onto the face of the girders instead of on top of them.

(1) I had to add many more footings because the township said each footing carrier too much load. They use a tributary loading calculation. It factors in the amount of deck over each footing which is half of the distance to the next footing. In your case, (5' + 2') x (3.25' + 3.25') x (40 live load + 10 dead load) = 2275. Our town assumes soil that can hold up to 1964 for 12" diameter footings. Therefore, for them to pass it I had to consider more footings, larger footings (2673 for 14", 3491 for 16"), or test my soil to see if it was better than their assumption. I didn't test the soil. 12" augers are common and hard enough to get 4' down. I wouldn't try bigger. That meant that I needed more footings. I added a beam for a second row of footing. For my plans, that gave me 1946 which was under the 1964 limit.
(2) I decided that it was desirable to get the beams higher off of the ground. That was important to improve the ventilation under such a long deck that was so low to the ground. Any ventilation improvement will reduce cupping caused by the bottom of the deck boards being wetter than the top. I changed my plans to move the front beam out to the face of the deck and mount the joists to the face of the beams. The second beam in the middle of the deck made that possible. Your plans for 30" high with 10" joints and 10" beams leaves a 10" gap between the beams and the ground. If you slope the deck away from the house so that the 30" is at the face of the deck, the gap under the middle of the beam could be even less that 10". That creates a pocket of air between the new middle beam and the house.

I wish someone told me this before I started.

Here is a link that might be useful: My Deck

The code says that footings closer than 5' to the house need to be as deep as the house foundation, which would be 8' deep in my case. That's why I only have one row of footings.

I have been looking for an auger and they all seem to come with 12" bits so I think I will have to do 7 or 8 12" footings instead. I didn't think about the ventilation problem, the decking will be 30' high but one end is blocked off by the a house corner. Putting the joists on top of the beam seems like there would be more room for error.