I doubt the problem arose from my driving habits, as I've been successfully driving sticks for 40+ years. Although after the problem got worse I did find myself sometimes going through shifting sequences without fully releasing the pedal, since that decreased the chances of popping out of gear when I needed rapid acceleration. I had been wondering if the clutch needed re-bleeding ever since they replaced both cylinders 4K miles ago; it felt like disengagement wasn't beginning until about half-way down. But since I got the car back yesterday the free play feels pretty much the same (and the tech report states that they felt the clutch was functioning properly). Shifting is noticeably smoother, less notchy, and I haven't been blocked out yet, or popped out of gear. I do wonder if re-doing the gearbox bulletin fix would smooth things out more (i.e., could the new cylinders and tranny overhaul have uncalibrated the box again?).
They tell me they think
the new synchros have been updated from the ones known to wear early. Does that mean the non-replaced synchros will go bad early? Mazda says, nah, they've mainly been a problem just with 3rd and 4th.
FYI, here's the service report from diagnosis and repair:
On test drive tech found hard shift into 3rd, 4th, 5th gears with a bad vibration in shifter in 4th. Tech performed transmission overhaul, replaced 3rd and 4th gears, synchros, and hubs. Also 5th and 6th hubs, retainers, and end bearing. Performed alignment and test drive, no faults found.
Thanks for the tip re tranny fluid. Will look into it. The Mazda invoice lists the fluid they used as "5W80-QT 75W80 Man. Transaxle fluid."