I think I got it?
Here’s the darp6 repo.
I ended up including the kernel specification at the module level alongside the ones that provide the system76 packages and configuration.
That let me remove the boot.kernelPackages line from my nixtop expression and replace it with the module in the list instead.
It also freed me up to use a more up-to-date nixpkgs for my system packages than the one that was used to build the kernel choice module.
LMKWYT!!