NextCloud migration was successfully completed yesterday! Two things made this possible: NextCloud's amazing administrator dashboard, with its active tips to fix problems (including telling you what command to run) and it's even-more-amazing documentation; and ChatGPT, which provided a correct solution to a database problem I have personally never encountered before and for which there was only 1, and I mean 1, Google search result.
Migrated our family users to new clients, too!