Ask HN: How to add some custom Android kernel code to my phone?

I currently use a Google Pixel 4a and I've got a few photos, videos, apps, contacts, etc; that I want to keep on it.

I've made a few little apps and things as various little experiments, and one of my latest ideas that I want to try out, actually involve some changes to the AOSP core; (specifically, the surface flinger)

My experiment is that I want to allow for a "desaturated but not fully grayscale" experience on my phone -- looking around in the settings, I can find grayscale, colourblind settings, or "Extra dim" - but not what I want. (and programatically toggling grayscale on-and-off causes hiccups in the display)

I think I've found where the code would go, and what sort of changes I would make, but how can i add those changes to the core/kernel and run it on my phone? would replacing the kernel mean I lose all my current app / settings (as this is my only phone and I want to continue using it with my settings)?

Any help or assistance on the above?