I tried the setup above and it didn't work for me.
I have deleted all reminders from the iPhone. Deleted all reminders from my Mac. Deleted all reminders from iCloud. Tuned off sync reminders in iPhone, and Mac system preferences. I signed out off iCloud completely on Mac and iPhone. Restarted, flushed Safari cache, restarted iPhone, and reset PRAM (Just in case). After all that I Opened Mac reminders, saw no lists, created a new list, logged back into iCloud on Mac, merged Reminders. Logged into iCloud. The newly created list doesn't show up. It's been about a day. All of a sudden a list I deleted from Mac and iPhone showed up in iCloud. I can't delete it. I created a new reminder in that list on iCloud and it doesn't sync to Mac. I logged into iCloud on my iPhone, merged reminders. 3 lists appeared titled "new list". One list contains reminders I deleted months ago. I don't know why they showed up. None of those list synced to reminders on my Mac or to iCloud.com. My shared Family list appeared on my iPhone (not my Mac) but show the title "new list" not Family. My iPhone has the sync list indicator (spinning wheel thing) and it hasn't stopped.
The short and long of it is nothing syncs.
At this point I am not sure what to do. I have read loads of articles on the community forums. I have even set the phone up as new.
Because this issue isn't fixed at this point I suspect it's so bad it's beyond the teams ability to fix. Or they don't care because they are on to iOS 14.