Never heard of a GoatLink before. Reading online, looks like it's primarily designed to improve shifting and wear performance with 1 x 11 systems. Sounds like you are using OK it with a double up front, being careful not to cross-chain, if I read your post correctly?

However, my understanding of the need for a Shiftmate is the different cable pull ratios between road and mountain shifters. Perhaps you don't have that issue using Di2? Don't know much about Di2, so could you explain further?

I found this article that explain pull ratios. Low Gear Range: Road Shifters & Gears For Easier Hill Climbing | CyclingAbout