Mail.app quits unexpectedly
Downloading and reinstalling macOS solved this issue.
The earlier unstable version of Mail.app was Version 13.4 (3608.120.23.2.1). The reinstalled macOS 10.15.17 now reveals Mail.app as Version 13.4 (3608.120.23.2.4). I suppose that this iteration, or perhaps some other invisible change, was responsible for the improvement.