Joined the Delta Program and did the console update for it but my dashboard didn’t change

The change in the dashboard is done by system updates. The dashboard is constantly changing and being updated. So all you could do for the future is to join the Insiders program and give your feedback during the testing
phase. But even then there could be changes which you won´t like..

Hi Volcannon,

Yes, you can rejoin after leaving at any point. It's possible that by joining and gaining exp so that you move up the preview rings to Delta at least so that you may get a future preview update where this issue is addressed and you can test it out for yourself
before those in the Omega ring or not in the preview program. Beta ring previews updates before Delta ring and the Alpha ring, where updates are tested first, requires an invitation to join.

Either way a solution to this will more than likely be tested in the preview program first. News regarding what is included in recent preview updates for the different ring levels can be found here:
https://news.xbox.com/en-us/insider/

Also here is an FAQ all about the Xbox Insider Program:
https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

In my understanding the Dashboard experiment, A, B, C selection, is totally random.

As "Both consoles say their OS Version is (19h1_release_xbox_dev_1911.191028-1335)" your consoles are either in the Beta or Delta ring.

Both those rings and Omega ring are getting a preview update News release notes Beta Delta Omega
Insiders today, which you can manually start or wait until it becomes mandatory.

After you do the update, if you want to change rings, it will be the perfect time to change as all those rings will be on the same release. Subject to eligibility.

Microsoft doesn't Delta Update.

A delta update is an update that only requires the user to download the code that has changed, not the whole program. It can significantly save time and bandwidth. The name is drawn from the fact that the Greek letter delta, Δ or δ, is used to denote change in mathematical sciences.

Seems you have to be invited to join the Alpha ring

https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

The current Xbox One Update Preview rings are:

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.