Some progress. I tried a similar thing on my wife’s Mac and iPad. On those, too, if I added a folder to iCloud Drive on the Mac it didn’t show on the iPad. But after I added some files to an iCloud Drive folder on the iPad, sync started to work in both directions. So I did the same thing on my iPhone and iPad - copied some files around - then bi-directional sync started to work. Had to do the same on a 2nd iPad linked to my iCloud account. Very strange!
So now GoodTask sync seems to be working. I do notice, though, that if I reorder tasks on, say, the iPhone, and GoodTask is open on the iPad, the tasks don’t reorder automatically. I have to leave GoodTask and come back to it, then the tasks reorder. I can see the _tasks.plist file syncs properly between the two devices (that’s the one whose modification time seems to change when you reorder tasks). Things just don’t seem to change by themselves. Is this by design?
Thanks for the help.