I just came across this on another thread her two days ago (my apologies to the author as I've forgotten their name). Buy a carabiner & clip the handle of your backpack to the rails on your saddle. This will give the pack an extra attachment to the bike, to keep it on. With this in place, you may be able to get rid of some of the extra straps.

you could also build a deck for your rack that would extend it a bit on both sides, that should not cost much $. You could use plywood (though it would eventually rot in the rain, etc) & just fasten it over top of your current rack.

I'd invest the $ I saved on a new rack into a coffee thermos that fits into a standard water bottle cage. Mixing electronics & liquids is never safe.

With this set-up, you should be able to use less straps to hold down the backpack, giving yourself easier access to the contents of the bag.