So we have the basic design, 5 keys, the basic layout, Pinkey, Ring, Middle, Index Front and Index Rear. We need to figure out what chord goes with what key. There's several standards, Engelbart's design, cykey, twiddler, but other than Engelbart none of them are designed expressly for 5 keys, and Engelbart was not optimized in any meaningful way. Like all good engineers do when faced with multiple competing, non compatible standards, I decided to add another competing non compatible standard.
I spent a few hours clicking in random combinations to try and figure out what fingers worked better with what combinations. I didn't have much of a problem where bending one finger caused another to bend enough to move a second finger, found zero issues with using the index finger to actuate the front, rear or both keys and found the hardest button to press was the pinky.
I decided to write a very small, very dirty python script to help provide suggestions. Each key was weighted, a letter frequency pulled off of Wikipedia and the permutations calculated and sorted by the expected difficulty of chording. The output was analyzed, tweaked and tested and the end result put in a handy reference sheet. I threw together a layout for the remaining symbols based off of a bit of practise. Time and testing will tell if this is the optimal layout.