Hmm, this seems like a solvable problem - install the kernel on disk, but don't promote it to the default kernel on reboot until all the DKMS modules have built

Hmm, this seems like a solvable problem - install the kernel on disk, but don't promote it to the default kernel on reboot until all the DKMS modules have built. At that point, you might even be able to optionally background the compile process and have it run asynchronously and update your kernel only once it builds successfully. Alternatively, teach GRUB to be able to boot "the most recent kernel with all of this set of modules available".