I was originally asking this in the hope my project could be workable with a kernel close to a generic Debian kernel plus a few patches.
Then I looked at the git history of the relevant kernel source code. The Raspbian kernel closely follows Greg Kroah-Hartman's branch, this is the official 4.19.89 kernel. Once reverts were paired with their target and both removed, there are 855 patches on top of 4.19.89. Debian's "4.19.67" kernel is a very distinct branch, it might have most of what is in the official 4.19 branch, but everything can in via very different branches. The most recent common ancestor with the Raspbian kernel was 4.10.
Trying to rebase those 855 patches onto Debian's kernel would be challenging. As such I plan to switch to Raspbian's kernel source for my project. Originally I had been planning to use Debian's "kernel-package" program, but that doesn't really like Raspbian's 4.19.89 kernel source. Turns out though that the Linux kernel has been evolving extra build targets. There are now "deb-pkg" and "rpm-pkg" targets, do `make bindeb-pkg` in a Raspbian source directory and you get something which behaves like a Debian package and will install cleanly on a Debian system.
This may be suboptimal for my purposes, but it accomplishes my goal (generating a packaged kernel which runs on a RP4 and successfully talks to the RP4 hardware).