There is a non-zero possibility that my "clunk" is not the bottom-bracket. It seems to go away when I stand up. The current bearings are fine. (In fact, they were smoother before I re-greased them, so I am worried I have done more harm than good. I haven't had a chance to re-test yet.) From what I can see, there was a lot of dirt, but it never got in as far as the bearings. It seems to spin more freely now, however. (I think it might have tightened itself up too much; at one point the lock-ring was a bit loose. Now I have erred on the side of a tiny bit of lateral play, which I may regret. Things I hate about square taper.)

Moving from Hollowtech II to square taper wasn't something I would recommend. I got it because WI 30mm spindle won't fit in my BB shell because a couple of Di2 wires loop through there. I'm seriously considering going back to Hollowtech II with the introduction of GRX (and 24mm spindles).