USB/IP for a KVM switch might still work, as long as termux supports all that

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.

Yeah, if they are old enough those might be removable? Then again, from that time period they might be slower than RPis...

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 🤷‍♂️