The only reason I can see for the DHA is to provide a pay rise without the company being seen to provide a pay rise. With that in mind, it makes sense for the Captain DHA rate to be higher than for FOs. Otherwise FOs would be getting a higher percentage pay increase than Captains.
FO dta of around $6 is laughable. Our time away from our families and young children is just as hard. We get just as fatigued. Our lifestyle is just as important. As per the other allowances, it should be the same as the cptn.
Would you also propose that the flight attendant DTA rate is increased to be the same as Captains then? Their time away from home is just as hard as the Captain's too. Or perhaps everyone takes a cut to be somewhere in the middle? We all know it just doesn't work like that. The other allowances you're talking about are agreed meal rates and have nothing to do with the value of your time.