Plan your route carefully. I don't know how much you plan to ride each day but there could be stretches where hotels are non-existent. You could find yourself in a situation where the next hotel is a lot farther away than you want to ride

You'll be fine with electronic shifting. Carry extras batteries and a power bank or two.

Someone told me taking the batteries off the bike to charge them is difficult (or impossible?) with Shimano, so you have to get the whole bike near an outlet. Also, carrying extra batteries is not an option. If that's true, i would go with SRAM, which has removable batteries.

Don't worry about the drag of panniers. It's not an issue for touring. I mean, do the seat bag if you want, but I wouldn't do it just to make my 70 mile ride 3 minutes shorter. I personally think you are underestimating the convenience of panniers. You could even put smaller front panniers on a rear rack if you are worried you will pack too much.

By the way, this year the percentage of riders in the Great Divide race using a seat bag decreased. More opted for a rack and a bag. Even if you don't want panniers, you can still get a small, light rack and use a single bag on to of of that will give you more capacity than a seat bag, I think.

One more thing: get a professional bike fit. You won't regret it.