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No, you don't need a 'very bespoke AOSP' to turn your phone into a Rabbit R1 — here's proof
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
The manufacturer has to release the phone's kernel source code before any custom ROM development can happen for the phone most of the time for that reason.
There is a reason that GrapheneOS only works on a couple of Pixel phones.
Google specifically designed the Android kernel so that the driver are excluded, unlike the normal Linux or Windows kernel, because, long story short, Qualcomm did not want it to happen.