I use (but VERY rarely, making bagels is a more involved process than other breads) a recipe for whole wheat bagels from the Peter Reinhart baking book - “Whole Grain Breads”.
As you say - just flour, salt, instant yeast and a sweet syrup - honey or barley malt is recommended. Water too of course, but that doesn’t count as a fifth ingredient.
Peter Reinhart encourages using a “biga” and “soaker” when baking. These aren’t “water roux”, but they will have a similarish effect of locking moisture (and flavour) into the finished items.