Definitely dry bags and ziploc bags inside panniers for important stuff to be kept dry.
I bought a pair of the Ortlieb gravel panniers on spec, to be returned if not suited. I tried one with a similar clothing load as I expect to take on tour I.e. one change of cycling gear, overnight gear, spare t-shirt, jumper and it all went in no proble I then to wash the kit I'm wearing on D1, to be ready for D+2. I also compare it to one side of my bananananana bags and its about the same.
That gives me
2x fork bags for tools and heavy stuff
2 x panniers for clothes and other general living kit, washbag, meds, book etc
Rackbag for food and valuables, routebook, easily removed
handlebar bag for bike top layers, arm waters, wind and waterproofs etc.
Seems like if I cant fit a 5-day b&b touring load into that lot, I'm doing something wrong. Next step, inveterate planner tha I am, is to work through in excruciating detail what goes into each of them