Which fight would be better?, Dubois vs. Zhang or Dubois vs. Hrgovic rematch.

I disagree. Dubois "won" the Hrgovic fight through headbutts. Hrgovic was comfortably winning had it gone to the scorecards. If he wins a rematch, so what, he officially won the first one. If he loses, that's two losses in a row, bad for his momentum.
The formula for beating Zhang has been shown. Good test if Daniel can follow a game plan, stay away from Zhang for half the fight and then turn up the heat late. Zhang could be cooked by this point, but is a good name on Dubois' record to get his momentum going again for when Usyk retires or vacates the belts to chase the money fights with Wilder etc.

 

Hrgovic vs Dubois would be better, but with an HONEST referee who stops Dubois from headbutting.

 

Dubois team should target "I can only bang for only 3 rnds" Zhang.

 

That's his signature move!
His team would probably grease some palms so Evander "Headbutt" Holyfield is the ref.