Well, if you're in a DIY mood, you could try the following.

- replace your panniers with mesh netting (preferably woven from stretchy accessory rope; this is the DIY part).
- Permanently attach these to where your four panniers would normally go.
- Put everything into dry-sacs; put dry-sacs into mesh panniers.
- keep everything you would normally lash to the top rack in a large dry-sac with a shoulder strap.
- bring an extra 140l bag of some sort.
- when you want to unload: put all the dry-sacs into your 140l bag, and sling it plus the rear rack bag over your shoulder.

There are a lot of combinations here. You could bring an 95l backpack instead of a 140l bag and squeeze the extra 45l into the dry-sac from your rear rack when you're dismounted. Or, you could bring a rear dry-sac that is normally half empty, so when you dismount it'll take all but say 40l, which could go into a smaller carry-on backpack.