There is a big chance you are already using this one, but I will write for who still has not find an appropriate keyboard for these round screen devices.
I tried different keyboard, and this one is by far the most comfortable to use.
You can switch between a full and a mini mode by swiping up and down on the upper bar. You need the full mode (a normal qwerty layout) to write down words that the keyboard cannot predict (passwords, websites and so on).
The interesting feature is the mini layout, that is a modified qwerty layout, with a very good word correction system. I use it in italian and english language, and I have no complaint about it.
For the best user experience, you should use the floating mode, and put the keyboard in the middle of the screen (in floating mode you can move the keyboard by dragging it with 2 fingers).
Also it’s better to reduce the keyboard height so that it doesn’t cover too much of the screen.
Also the gesture system is very comfortable. If you swype right you type space, left backspace, right up enter, left up you have the voice input.
The trial version lasts a month.
p.s. For those who have a single-touch screen and cannot move the keyboard, I uploaded my settings files. You need root to replace them.
You have 4 xml files in the zip. Open a root file explorer, put those 4 in the folder /data/data/com.whirlscape.minuumkeyboard/shared_pref, replacing the default ones. Verify that their permissions are -rw-rw----. DO NOT replace the folder, just the files inside it. Now reboot.
You will now have the floating keyboard in the middle-lower part of the screen.
I suggest to install an alternative app for the back button, because with this keyboard layout the default swipe command will be pretty uncomfortable.