Sanji already is the most durable in the crew alongside Luffy
But you mean increase durability in order to increase his flames power?
Well, if we stick with the fact that Sanji needs a tougher body to boost his flames, only CoA can do it from now on
BUT Oda could simply come up with something else to make a new Devil Leg version, like a much tougher CoA, Adv.CoA, CoC coating, or something else Sanji finds out about his new body, something like "if i take more damage, my body regenerates tougher than before", then Oda could make a fight where Sanji needs to be beaten like crazy, almost to death, as an strategy for him to increase his flames to another level and destroy his opponent.
By default, i can pretty much tell that Sanji with the Exoskeleton is above his siblings, it works better on his body (im making a thread on that)
Perhaps he doesn't even need another Devil Leg flame tbh, i only think it might happen by the EoS fights, when he fights an Admiral or the strongest crew out there.