If you were to visit a leath goods supplier, you would find the best grades are most often a matte finish. The rear seats may have had a sheen originally, but that is not a good thing imo.. Meaning, although tanneries have methods to coat or polish the stuff, the point of leather should never be in the direction of duplicating vinyl, which is clearly a less flexible, imitative, substitute material. Everything oxidizes, and so will a matte finish, in a few years.
You're 90% of the way along, that if the rear seats were done now and matched the front. you would have an award winning interior.