Success! (I think...) I added 4 oz. of ATF and there is no more slipping in 1st. On the dipstick, this brought the fluid level from about 1/3 of the way between the min and max (on the "hot" 80C scale) to just a hair below the max mark. Now no more slipping at all, even when I tromp on it from a full stop. Throughout all of this, I carefully checked the fluid level at hot idle in park, first by pulling the dipstick, wiping it with a lint-free cloth, putting the dipstick back in, then out, and checking for the lowest spot on the dipstick that had an obvious dry spot (this is my normal procedure of many years).
I drove it all today around town again on errands, and not a single problem.
Operated perfectly. Even when cold first thing in the morning and then again mid-afternoon after the car had been sitting most of the day. Even when trying purposefully to get it slip with quick starts, tromping on it while rolling stop signs, trying everything to get it to slip or chatter.
My theory is that with an older transmission, possibly with a worn or sticky B2 or worn main pump, the tranny needs all the fluid it can get to operate properly, without going over the max mark which would start causing other issues.
If this didn't fix the problem, I'll post back, but I think this has got it licked. Guess everybody was right, this transmission (a 722.369) is very sensitive to fluid level, and apparently even more so as it ages.