I did try 5.1.4 and it killed my CPU also.
The 5.1.2 one I made works fine though in terms of CPU, unless there is some sonic problem with it because I don't know anything about ambisonics...
The LogicPro CPU is getting hit by having too many channels per plugin being processed. With 100 tracks of that, it turns into 1200 audio channels at 7.1.4. Regardless of the fact that using a downmix reduces how much audio is actually being transmitted on a subset of those channels, it brought my system to its knees.
Its nearly cut in half with 5.1.2. So it makes a big difference for me here. My home atmos system is also 5.1.2, so I don't find that format to be that odd at all!
Of course I can also just use 5.1. Or even Quad! In the quad case, you didn't provide a 3D-to-quad downmix...there is just the original mic array for quad...but may be ok if I wanted to do that, or 5.1 if I want to use your improved downmixing. Eventually I will setup quad loudspeakers in my studio, but I'm not likely to add center channel or LFE in there.
But I quite like the sound I am getting from 5.1.2 through binaural headphones, which is the best my machine can handle.
I didn't find it that difficult to use the AllRADecoder... Since you are unable to comment on what I did, I will just have to be ok with it and stop wondering if I did anything stupid. ;-)