An image-based system would be the bare minimum to achieve basic security, but there would still be so many security issues compared to Android and iOS, that I don’t think Linux phones are worth putting time and development effort into.
AOSP is a fantastic base for open source mobile systems. The FOSS mobile development community should rather shift its focus to AOSP, develop a good understanding of it and get familiar with the code, instead of trying to reinvent the wheel with mobile Linux distros.
I’m talking about developments such as postmarketOS, Ubuntu Touch/UBports, Phosh (mobile GNOME), Plasma Mobile, etc.
I see so many people here on Lemmy who are desperately waiting for Linux phones to replace their iPhones or Android phones, without realizing that idea is absolutely utopian and unrealistic.