Actually, Jim suggested concatenating the commands, and I couldn't get it to work. And just now, I tried it again and it's flying! Pressing one F key builds the exchange in the memory slot, and then sends that slot off to the radio's keyer. Not sure what changed (maybe baud rate? cpu load? who knows). Bottom line: we have a winner!
This scheme will probably work for quite a few Yaesu rigs (also FT-991, FT-2000, FT-3000, since they have the same CAT command set).
Jim if you're seeing this, thanks for your ideas.