Github link: https://github.com/Dakkaron/Fairberry
Here's a video of it in action: https://www.youtube.com/watch?v=iDb8_ld9gOQ
I've been using it for almost two years now, and I'm not going back.
It's based on a spare Blackberry Q10 keyboard and a custom Arduino-compatible board that reads the keyboard matrix and outputs it as USB HID to the phone. From the viewpoint of the phone, it's just a regular USB keyboard, so no special software is needed.
But I do use a custom virtual keyboard to have just two rows of symbols that are not natively on the keyboard, as I didn't want to add another layer of rarely used symbols that I'd have to memorize.
(On the image you can see Ubuntu with XFCE4 running on it. I chose Ubuntu because it's what was easiest to get running in a chroot jail on the phone. I'm using VNC to display the GUI. I even managed to get FEX (x86/x64 emulator) and Wine running, so it runs x86/x64 Linux and Windows apps.)
it works on android 13 (as they're still targeting android 9, so new limitations don't apply to them), but you need to disable phantom process killer to make it work in background. (you need android 13 or newer or root to fully disable it, it can only be disabled until reboot on android 12 unless you have root) both chroot (with root) and proot (slow, no root) are supported. Termux-x11 is still kinda experimental though...
Thanks for the info, that's good to know!