Has anybody considering using TCP for flow control, instead of the various cobbled-together line and character counting protocols? Flow control is a solved problem and the name of the solution is TCP/IP, with serial transport layers like PPP and SLIP.
I realize that TCP/IP/PPP is too heavy to fit on small AVRs and thus could not be considered for grbl, but for g2core-class processors, it should fit easily.
Using a mature standard technology would solve the buffering problem once and for all.