It’s not that hard to cut a few strips of wood and glue them together to make a simple picture frame.

Even if you don’t have the materials or woodworking tools, go to the local market and buy a bamboo clipboard, then rip it into suitable strips with a knife. This is literally kids’ arts and crafts level stuff – and people are instantly jumping to use a 3D printer to make objects that look like cheap plastic crud.

This. Any old phone can connect to an ESP32 or even ESP8266 configured in ap mode. Make a shortcut on the phone’s browser to 192.168.4.1, and bam. I do this regularly with an old Galaxy S5 that doesn’t have phone network service any more for various troubleshooting and minor control purposes. Actually makes it worth keeping the old S5 in fresh batteries since the back pops off for easily replacing them.

Just make sure the adapter or splitter cable specifies data (as usb host) while simultaneously charging.
For integration like this, USB 2 data rates (480 megabits pr sec) are sufficient, and most USB serial adapters usually support just 12 megabits pr sec anyways.

Most phones of todays and last five to seven years, use USB-C with USB PD for charging. PD – Power delivery, makes it possible for a device to be set to be USB host while charging. All Samsung Galaxy since S9, probably also the S8, support this. I use this feature every day.
To make it work, one can either buy a powered USB hub with HDMI and/or Ethernet connector, but they cost a few tens of dollars, or one could buy a simple and cheap USB C splitter cable with PD support from eBay or Ali Express.
The handling of usb devices is wonderfully simple to program in Android apps, and there are several GitHub libraries for handling a variety of different USB RS232 chips.

But ok. Realistically? We live in 2024. Just buy an ESP32 for a few dollars and use Bluetooth Serial Port Profile or BLE.

Or WiFi, why not? Those old phones all support WiFi. You can put your whole app in the ESP32 as a web-based application and use your old phone’s web browser to control your peripheral. As long as you only use basic HTML, any old browser on any old phone is going to work.

And then you also have the connector free for charging.

Hehe, I was about to write something like this too. Some phones don’t have USB OTG, like the iPhones. But specifically the old iPhones with the 30-pin dock connector, have audio in and out on the connector. You could quite easily implement an FSK protocol to talk to peripherals.

With the new iPhones it’s harder, they don’t have analog in and out anymore, lightning is all digital and you need an Apple-provided chip to authenticate your peripheral, or the iPhone will immediately ban it.

You could actually still go the ‘acoustic coupler’ way, using a phone’s speaker and microphone. But I guess the screeching sounds would annoy anyone, including yourself, within a few seconds.

Maybe you could modulate data on top of an audio stream. E.g. use modulated Amiga mod music to transfer data, so you can listen to Amiga mods when controlling your peripheral. ;)