Love your setup with Wolfman side bags also- are they stable on the sides like that?
Yes! They basically have a stradle strap that goes over the top of the seat, then a lower strap that I attach to my passenger pegs. On the ride in that photo, I did lay the bike out on the right side at a relatively high speed in the dirt (3rd gear, not sure speed) - and there is really not even a sign of abrasion, and the bags needed no adjustment. Lots of water bar jumps, rough rocky roads and challenging terrain, I hardly noticed the bags. They are pricey, but the ability to run without a rack is awesome, they are waterproof, and seem to hold up very nicely. I admittedly have only used them on one other trip, as I do tend to go with a top bag first, then add side bags if duty calls.
I was thinking a US10 would be a good size for what I'm after but the only thing I don't like is the idea of reaching into the bottom of the bag from one end while its strapped to the bike to find something small eg a leatherman, chocolate bar etc. it's probably not as bad as I imagine though.
And then a US 20 would be a good longer trip size.
I don't have a US10 you've gotta make your own decision there - but when my us20 isn't packed tight, it's still nice and secure as the synch compresses the load, whatever size it is.
And I have a lot of experience with working with dry bag type bags, so I've picked up a lot of tricks to make dealing with one large space. This includes having smaller bags within. as it pertains to DR camping, I have separate bags for clean clothes and dirty clothes, I have a bag just for camp kitchen, I have a toiletries bag, etc. etc. I also make it a point to put more frequently used items either in my camelbak, or near the top of the roll-top. For the most part, the US20 is filled with stuff I won't need till I get to camp. I run my camelbak as light as possible. My tools live in the stock tool tube. My flat kit/spares live in the US5, front tube, patch kit, etc all live in my fender pack. By planning your pack, and making subtle changes over time, I think you will learn to make great use of a dry bag.... And your stuff will stay drier than they could in a zippered multi-pocket type bag.
I actually just placed an order for the Dowco I posted earlier so I'll post pics when it arrives. I mocked up a cardboard box the same size, sat it on the bike and filled it with the stuff I normally carry on the trail and the size seems spot on.
Then I reckon a US20 or roll top duffle bag would be a good buy when more storage is required.
The kriega bags mate together - so they are a great expandable system. If you got the US10, and then two US5's, you could mount the 5's to the side of the US10 bag and essentially have a US20, but divided into three. Or get the US20 and hang 5's off of it, or a 10 on top of it...
Like most have said here - lots of great options out there, no wrong way to do it - though I would say bungee-cording some anonymous cheap bag is probably my least favorite option -I've had to stop and pick up other people's stuff as it falls off their bike, and wait for them to re-secure it enough to know I don't want that for myself. Early on in my DR's life, I took a trip where I took a large back pack that I strapped to the seat using moto- tie downs between the grab handles. This kinda worked, but I was checking to make sure it was back there often, and it did move around and slip. I later added that padded/meshy rubbery stuff that your wife puts in the silverware drawer between the seat and the bag to help it grip, and that helped a lot... But I would never ever even consider leaving the house with a bungee holding the same thing in place. YMMV