Linux

Are there mobile desktops for Debian?

If you just want the DE and to configure everything else yourself, you can always just install phosh or sxmo on top of a debian/raspbian installation.

Mobian/Droidian are targeted for smartphone SoCs, so they would take a lot of tinkering to get runnin on an RPi

If you want a full OS that's already configured to be a smartphone-like device, something like Glodroid may be your best bet. They're an infrequent updater, the only reason I mention them is because they can target Broadcom devices (like RPis)

If you don't mind getting away from building the hardware yourself, and just want a phone that you can run linux on, FairPhone/PinePhone/Librem 5 seem to be the way to go

Aside from that, afaict you're in for designing your own device from extant components and then crowd-funding to pay for a factory line to assemble it for you (this is essentially what the PinePhone did)

(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!

why not a mobile Hotspot from XYZ carrier and then a purpose built wifi only Linux device /existing Linux phone?

I think open phones like fairphone or purism's device or pine phone exist with more refinement already. doing this seems fine from a tinkering aspect but not at all close to daily driving.

I think there are 3 reasonable options (that I'm pursuing currently):

Linux on a phone designed for Android

I've tried a few different models of phone. I'm currently running mobile nixos on a OnePlus 6, but waiting on Plasma Mobile to be fully functional. When I last played with it a couple months ago, the virtual keyboard was broken. Nothing else I've tried has had all features on the phone work.

A small Linux laptop with a mobile network card

I am trying to set up an MNT Pocket Reform as a phone replacement. The hardware is almost there. The software and firmware have a bit of a ways to go, but it has promise for a subset of people.

A Linux-first phone-like device

There are some phones that are set up with PostmarketOS or other mobile Linux distributions from the manufacturer, but they right now tend to be really pricey, under-performing, and don't really have upstream support. The Mecha Comet seems to be another interesting option. I've pre-ordered one and I'm excited to see what it's capable of.

What carrier do you plan to approach to use this on their network?

This is awesome!