I'm very happy with my cheap 16 channel FrSky Taranis X Q7 so far. It comes with the option of self centering on both sticks just by turning a screw. It's definitely not as user friendly as many other radios, but almost everything on it can be programmed.
As you have discovered, the Tamiya MFU's were designed for older radios with analog stick trims, but can be used by utilizing the dual rates feature of computer radios with digital trims. DX6 should be no exception.
Here's the thread for setting up dx6i with mfu
http://rctruckandconstruction.com/showthread.php?t=3535
And here's a couple of video tutorials
https://www.youtube.com/watch?v=jAaf0FtS6Ms
https://www.youtube.com/watch?v=lpez59uvSzc
Disclaimer: I do not own an MFU