Four kongs is all the same toy regardless of filling.
If that many 30 min walks isn't wearing out the dog, put a jacket on her and give her two one hour walks. Or one good half hour RUN. Or find a pool you can swim her in, they're out there. The point is to tire the dog and walkies is insufficient for many many dogs.
Large raw beef bones my present enough of a challenge to keep her busy.
Large beef pizzles also work, or large woven ones.
Obedience classes tire dogs and work on the relationship.
Sports offer something to do.
Putting a stressful dog in a crate with one that isn't, that is a pup, encourages bonding to each other and less listening to you AND the pup will pick up stress behaviors from the older dog.
When I had a particularly bad rehab we once hung a bike innertube, from a bungy, screwed into the ceiling. Having a tug toy wore her out. Always take the metal part out of an innertube before using it.
Even placing a low board across door frames, size depending on the size of the dog, can change things up and add interest - and ... tire the dog.
You might consider finding a place for her to go and play all day with rowdier dogs, a friends house or a day care.
Venting stress requires activity either chewing, or playing or learning, or something new.