Add the ability to add a dashboard to a menu link

Several things in one single issue!

> But it is not directly apparent or is it even impossible at this point, to choose which dashboard to use on admin/dashboard

Users will see the first one that is enabled, they have access to, sorted by weight. You can sort them in admin/structure/dashboard.

> having the ability to add another dashboard via the ui?

If you mean having different links in the new navigation, no, you can't on the dashboard block. But you could add a menu link to one of the several menus navigation is using, it just won't be on the top section. I feel like showing all the dashboards there would be quite overwhelming in most cases. Once navigation APIs stabilize we might be able to explore more options.

> it looks like #3486246: Provide a block for Navigation top region on installation is adding a block similar to Drupal CMS for that purpose.

Yes, that region is protected, and a block can only be added by code. The issue you mention will add that block on installation, now that there is an API for that (remember, navigation is still experimental). That won't be available until 11.2.0 is released though.
The workaround is what Drupal CMS does, providing navigation.block_layout.yml in a recipe. If you feel comfortable editing that by hand and importing it, you can do that in your sites.

> I wonder if it would be a reasonable step to either add a select field or an auto complete field, depending on the number of available dashboards, to the edit menu link pages

You can always use a menu link with the dashboard path? Or am I missing something here?

Also, we recently added ✨ Provide integration with coffee module Active for coffee integration. So if you are using coffee module, you will be able to Alt + D, write dashboard, and see the dashboards.