I followed the instructions in that post quoted above.
Think I managed to do as suggested.
Stopped syncing old DT3. Installed DT3 on new Mac. Copied DB’s in existing file/folder structure across. Copied the entire container directory (to bring the settings, preferences and inbox over), then started DT3 again. Verified. Checked. Switched on sync.
All appears to be working. 89 odd files not syncing across from DTTG — not sure what that is about, and if related. All other DB’s working aa expected, with no errors. Just the Global Inbox causing issues.