I don't think dw would publish articles without a reliable source. Also welt.de is the online-presence of "Die Welt" (the world in english), which is a major german newspaper. I would say that it's not known for world-class journalistic results and major breakthrough investigative journalistic pieces, but it's still a reliable, conservative newspaper that does solid journalistic work.

It is. It’s the nationwide conservative quality newspaper in Germany that regularly conducts investigative journalism.

I think it is reliable. Many German journalists report about that topic, e.g. Tagesschau: https://www.tagesschau.de/inland/corona-impfstoff-deutschlan...

For whatever its worth, the Guardian picked the story too:

https://www.theguardian.com/us-news/2020/mar/15/trump-offers...

It isn't. It's the big brother of the tabloid 'Bild', from the same publisher, rephrasing typical right-wing populism and fear-mongering for readers who like to think they are intellectuals.