Are you sure you can't create a custom mix on the DX6i to do that? I guess not. I looked at the DX6i manual and it says "(THROTTLE cannot be mixed with itself or as a slave)."
Not sure what it is you need to do but the FlySky i6 can create a master slave mix where CH4 elv affects Ch3 thr. The i6s mixing is pretty limited but you get 3 mixes to work with. Not sure about hold/pitch - sound like heli settings that I'm not that familiar with. It has idle and a throttle curve in airplane mode and hold and pitch curve in heli so probably. How the mix will affect that I cannot say.
You would have to be more precise about what you want and I might be able to try it.
I'm sure any OpenTX based radio (9XPro, Taranis Plus or flashed 9x) can do it. You could download OpenTX companion and simulate it to see if it does what you want. You just watch the channel display to see what the channels do when you program them.
My guess is you could also do it on the cheapo
HK/FS T6A and
data cable if you want something really cheap to experiment with. Download this
Digital Radio software to see what you can program with it. I think it's actually a bit more flexible than the i6 for mixes if you can put up with the rest of its inconveniences. Not sure it does the Throttle and Pitch thing though.
None of these radios is Spektrum compatible so you will need to use a different receiver. There are ways you can make the OpenTX radios Spektrum compatible but you have to spend some money and the solutions are not that great.
Otherwise I suggest you download the
DX6 manual and see what it can do.