Step-by-step instructions to build a smartphone that is open-source, upgradeable, repairable, and Big Tech free. - GitHub - evanman83/OURS-project: Step-by-step instructions to build a smartphone t...
IMHO way too big to be usable, but it is an interesting experiment of what can be done with off-the-shelf components.
I think you can flash some android OS's on the rPi, OmniROM is listed as an option, so it could be a better starting option instead of raspbian - https://omnirom.org/