Dry bag for the tent? Not sure that is warranted. If it is raining when you take it down, you might want a plastic bag to keep the wet tent from getting everything else wet, but otherwise I do not see any reason for a waterproof bag over the tent. I do not even bring the sack that came with my tent, I just roll it up and put a strap around it so it does not unroll.

Think about what size drybags will fit in your panniers. Too big of a drybag and things won't pack well. The diameter should not exceed the thickness of a pannier. I have two small compression stuff sacks that are extremely useful, but not dry bags. The compression sacks have a diameter that is about the same as the thickness of my rear Ortlieb Classic Backrollers. When I have the compression sack fully compressed, the bags fit perfectly oriented horizontally on top of everything else in the rear panniers, one compression sack in each rear pannier. They are the last thing to go in each pannier. I put a sleeping bag and some clothes in one, clothes in the other. The compression sacks are one of the most useful things I have for packing.

I also have a Ortlieb duffel that I put on top of the rear panniers, medium size, I think 31 liters. Length of duffel is about the width of the outside of the rear panniers.

If I did not use Ortliebs, I might use a dry bag for my sleeping bag, and maybe one for some clothes. I use a very small dry bag for first aid kit, meds, things like that. Another very small dry bag for electronics. But other than that, just about everything I have could get wet without ruining a trip. I still would use plastic bags for a lot of things, but I would not put a lot of stuff in drybags.