Ok, I thought I learned the lesson: Do the taxes early. And I did. All done, check sent, check received, check!
But the bigger lesson is to get the OTHER taxes done early, too. So that includes my mother-in-law’s taxes and those of any beloved family members who need my help and may end up against the deadline.
Now: All done!
It seems to be less awful when done early. Will try to remember for next year.