(Personal opinion not based on scientific evidence.)

I would say that’s not possible for a hobbyist. The main issues with this kind of DIY phones are performance, compatibility with existing software, and most importantly battery life.

The Raspberry Pi was never designed to be used with a small battery while still staying connected to the internet to receive notifications all the time, like a smartphone. If you want to build a usable smartphone you will need an efficient co-processor to do these tasks. This could get complicated fast unless you use a CPU designed for this job.

There has been some interesting progress with desktop environments and small touch screens. Still nothing as good as Android, but nice UI is no longer the main issue imho.

As for compatibility, good luck running Signal, Matrix and Thunderbird in the background without draining the battery.

That being said, if you are just doing it as an excercise without expecting to build a device that will replace your main phone, you can definitely give it a try. Have fun and learn much!