I've used backpacks for short riding, I think about 100km has been the limit. They have been lightweight, with no stiffener.
There were three issues -- any hard object in the backpack always found a way of boring its way into my back muscles or spine. The sweat. And on longer rides, I inevitably had lower back ache.
Personally, I wouldn't ride with a backpack for any long tour distance. On a 1200 randonnee, Machka used a Camelback. After about 900km, she was unable to use her arms properly and was in great pain, even though it was empty at the time I removed it from her. I know it's an extreme case, but the cumulative effects are something you might need to consider.
Personally, I would just invest in a Carradice Nelson Longflap. You can put most of you load in there, and strap the remainder on the outside. You will probably need the Bagman support to attach to the seat to stop the Longflap flopping down on the the wheel and into your thighs as you pedal. It keeps everything together in one, manageable package which is handy for security, is waterproof, and if you take into account the combined weight of your current rack and drysacks, the Longflap and Bagman support probably come in about par.
Incidentally, there is an alternative to the Bagman support that would make the combination much lighter, although a little less stable. It involves suitable-sized plastic electrical or water conduit cut to length to act as a spacer to keep the bottom of the bag away from the seatpost. The conduit can be ziptied through two holes to the seatpost, and similarly to the loop normally used to attach the bag to the seatpost. It needs a little fiddling to get the length and security of attachment right, and the seatpost end of the conduit could do with a little shaping, but it worked very well for me with a Brooks bag I have.