If the release is exactly the same, I’d just use the earliest release date. I think this is commonly agreed on.
Musicbrainz doesn’t focus on which dates a release is available on different stores. Maybe in some cases that information could be valuable and if you think so, feel free to add it to the annotation.
Edit: For physical releases, a repress of an album could require a new release on MB even if the album art looks the same.
Edit2: Replied in the new thread.