The problem with the FD is that when shifting from the large chainring to the small, it makes a large move instantly, but it's not enough to move the chain to the smaller ring. After about 2 - 3 sec, it makes another smaller move and the chain does shift to the small ring. I tried adjusting the derailleur so that the first large move will make the shift change, but then when shifting the chain from the small ring to the large ring, it won't make it. In frustration, I used a pair of pliers and squeezed the metal derailleur loop together so the chain barely fits inside, made adjustments to the positioning, and voila, it works. There is some light rubbing of the chain on the sides of the derailleur loop.
If the chain's rubbing is too much, I'll pull the cord on a 1X12 system. In which case, should I go with Sram or stick with DI2, since I'm already all in on Shimano Ultegra? The Sram Rival etap GX AXS Mullet (plus chain ring), conversion looks very reasonable in price and easy to install as it's wireless. I can get a 12 gear 10-52 freewheel, which gives more range than the Shimano offerings. For Shimano, I'd have to buy an XT M8150 derailleur and freewheel, along with the single chain ring. The freewheel would be 11-50. Seems Sram gives a better range and I like the wireless idea.
Edit: Actually, I discovered you can get freewheels from both SRAM and Shimano with good range. SRAM: 10-52, Shimano: 10-51. That makes the decision harder.