Yeah, it could take a few hours (or even days) for all mirrors to sync across the world. 
Changing the server in Software & Updates to point to Main server 
and refresh your sources might accelerate things.
At this time, it's okay to use the -d flag since 20.04 was just released and this just bypasses the check to stop users upgrading to what would normally be a development branch.
If I remember, users on 18.04 won't see an invitation to upgrade to 20.04 until the first point release is out (20.04.1) in a few months. It could be a few days for 19.10 users to see a notification (or do-release-upgrade without the -d ), in case there is a serious issue.