Flour – technique for making larger batches of roux

Typically, you go for 1 tbsp flour, 1 tbsp butter for each cup of liquid. You may have to adjust slightly, but that's a good starting point.

After the quantity of roux, the next factor to thickening strength is cooking time. The longer you cook the roux, and consequently, how dark it gets, the less thick the final result will be for a given quantity of roux and liquid.

For a white sauce (i.e. Bechamel and Bechamel-based sauces) you want the roux to stay more or less white. For a Gumbo, you cook it until it's light brown (sometimes called red).

A roux is a stable mixture (an emulsion) of fat and water held together by an emulsifier such as starch.

So you could try any number of flours from grains that contain starch such as potato, rice, barley, buckwheat, etc.

As far as the Maillard Reaction taste and color that you'd like to substitude, potato and barley (IMO) are the better bets. The note on arrowroot may be that it doesn't brown through the Maillard reaction.

An approach I would suggest is to use more than one flour/emulsifier and try to achieve consistency and taste by varying two or more ingredients as opposed to a direct single substitute. This may also mean that you'd likely have to the color/taste/browning first and then bind it to achieve the final result.

Have a look at Bob's Red Mill's list of flours and experiment. A little bit of Hazelnut flour and you might never look back at an ordinary wheat flour roux again.