How else can I incentivize channel partners besides direct cash payments?

Understand their priorities. It’s not likely they make much money from margin on your products because others discount them so severely and they must be competitive. The best partners figure out how to wrap their own services around your products and drive profitable revenue from those services. Any assistance you can provide in the way of reusable enagement models, promotional ideas, assistance in promoting their services around your products are their most prized gifts. Cash is good short term. Support growing their services business has long term beneficial impact.