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.)
The original reason for this was that I didn't want to spend all that time getting pass-through charging to work, so for charging the keyboard needs to come off.
But this "quick release" design has the advantage that you can use the phone without keyboard when that's advantageous, e.g. when I want to type really quietly while in bed.
This also gives me a direct comparison between physical and virtual keyboard, that you usually don't get on a pure keyboard phone. And I have to say, typing on the screen is a PAIN compared to the physical keyboard.
For the two virtual rows? Yeah, I'm using the Gr8ly customizable keyboard for that (https://play.google.com/store/apps/details?id=symbolsshortcuts2.gr8ly.co.za.symbolshortcuts2) which allows me to put whatever symbols I want on there.
Thanks!