Last night I took a sheet of paper and redid the calculations like I'd done three years ago, and changed one of the shift points in my synchro shift settings. I'll test ride it today and see if I like it, and if not, I'll revisit the topic again. There's something about the difference in feel of the upshift or downshift that I'm not liking, though my mind is hazy right now so I gotta go try it again to solidify my thinking on it. It may just be the difference in feel between gears using the small ring and gears using the big ring, I don't know. With my current 12-25t cassette I've got mostly 1-tooth shifts, since it's 12-13-14-15-16-17-18-19-21-23-25, but my current synchro shifting is waiting all the way until I get to the 23t before shifting suddenly back to up to one of the higher gears but using the small ring. That means I suddenly go from the realm of 2-tooth shifts back up to 1-tooth shifts, so the change in rpms in these shifts changes. I'm also not happy about how cross-chained I am if I'm still on the big ring and the chain is going back to the 23t cog. I may set up synchro to shift if I move down from the 19t, so it'll go to the small ring and then back up to the smaller cogs with the 1-tooth shifts. It would result in a better chain line, and also keep me in the realm of 1-tooth shifts for all conditions except climbs steep enough that in the small ring I end up in the 21+t cogs.
I'll have to unwrap my left handlebar this weekend, remove the Di2 cable that is probably broken due to being held in tension at a high angle going into the junction box, measure it, and then order two replacement cables in the next size up. I believe they come in increments of 50mm. When they arrive I'll replace the cables on both sides at once, since the right-side shifter's cable is also at a pretty sketchy angle going into the junction box, so if the left cable went bad from this it's only a matter of time for the right side cable to go bad too. May as well fix them both right, then I only have to rewrap the bars once. I'll re-wrap the left side this weekend using the same bar tape just so I can keep riding while waiting on the replacement cables, but I'll probably start with new tape when I do the actual cable replacements.