• Connected standby is already somewhat possible (it’s actually done on the hardware/firmware side on most phones), it can work with something like ntfy with a fairly simple script IIRC
• We have sandboxing and permissions figured out pretty well with Flatpak (there are improvements to be had for sure, but all the basics are there)

The one main remaining barrier (apart from thousands of paper cuts everywhere and lack of apps), is indeed process lifecycle management. It’s the most complicated one to do, because in order to work well it requires apps to cooperate in some way, either by completely and honestly shutting down when not doing any work, or by providing ways to check if there’s any work to be done without running the rest of the app, or both ideally. None of the apps currently do that, so the only options are (1) just let apps do whatever they want, draining the battery, or (2) send SIGSTP to apps that are not in the foreground, losing background notifications and such.

I’m sure you are already aware, but just in case, there’s a lot of prior work in getting a truly Linux mobile phone.

There are ready-made devices like PinePhone (the PinePhone Pro looks the most promising one of the bunch), Librem 5, and Liberux Nexx. I think at least some of those companies publish schematics for their boards, you should probably check those out if you want to design your own.

There is also another direction, taken by postmarketOS and the like, to install Linux on a phone that shipped with Android out of the box.

It should be easy enough to install postmarketOS on your device, since it seems to have support for raspberry pi. The benefit of postmarketOS here is that it makes it really easy to install mobile Linux UI shells, like phosh, gnome-mobile, plasma-mobile, or sxmo. This will let you try all of them out and maybe pick one as a starting point for your software stack.