The value proposition of old or used android phones as SBCs is insane! You've probably got some in your drawers, or can at worst buy some carrier locked ones for 30$. You get a device with better compute than a raspberry pi, with a screen, cameras, speakers, flashlight and battery attached!
Personally, I use them to run and monitor my 3d printers.
In my experience using an older android phone for self-hosting, the downside was a lack of ports and I/O. Being stuck with a single USB 2 interface and wifi was less than ideal for my use case.
Imagine the single USB C port being 40Gbit/s or even 80Gbit/s (USB 4 Version 2). Given a nice docking station and some additional enclosures, you could technically even connect hard drives and run the phone as a low-power NAS. Or/and as a multimedia station for your 4K TV, I mean the integrated GPUs are usually more than capable enough.
A bummer that they stick to USB 2 speeds, even for most high end phones.
I don't have a particular guide at the tip of my fingers, but I can share some recommendations based on my experience:
prefer a phone with USB-c if you plan on connecting USB things to it. the otg adapters for micro-b are kinda hit and miss when it comes to keeping the phone connected to power as well.
look out for clearances of those carrier locked prepaid phones from physical stores, you can get nice devices for nearly nothing
whatever you're running on the phone, make sure it starts at startup, so you don't need to go launching everything if you reboot for some reason
if the phone is"mission critical" e.g. random restart while in the middle of a print is unacceptable, turn off all the automatic updates and such.
a VNC server has been helpful, to remotely poke at the phone if I'm too lazy to go do it physically
get something that'll keep the screen off the phone on. I've encountered reduced performance regardless of what battery optimizations I've turned off without doing that but YMMV depending on ROM.
I fully expect the screen thing and the batteries bring in there constantly charging to kill the phones I'm using eventually, but it's something I expect and accept. my octoprint phones have been fine so far, for a bit over a year 🤷♂️