Could you help me understand your reasoning behind not pairing up with or taking advantage of existing projects like Halium or LineageOS?
I'd love to run mainline Linux without any Android cruft on my old phones, but realise that this would take a lot of effort per each device. Each Android device is different, with its own kernel patches, proprietary modules and sometimes Android-specific userspace HALs. The Tegra in the Nexus 7 was designed to run non-Android Linux, there are native X11 drivers for that thing - that's not the case for devices like my current phone.
I'd love to run mainline Linux without any Android cruft on my old phones, but realise that this would take a lot of effort per each device. Each Android device is different, with its own kernel patches, proprietary modules and sometimes Android-specific userspace HALs. The Tegra in the Nexus 7 was designed to run non-Android Linux, there are native X11 drivers for that thing - that's not the case for devices like my current phone.