To tell the truth, I rarely find myself needing to resupply mid-chapter, since I usually prepare characters with the necessary items at the prep screen, and few chapters can wear out any weapons that are either replaceable or durable. Convoy space running out always seemed really pointless and frustrating, personally. I think mid-chapter convoy access will continue to be largely as relevant as it usually is, namely, only used in case of emergency, unless FE14 has a great level of divergence from Awakening-style maps.
I do like the idea of a convoy unit, at least conceptually, although, in anything barring a defense chapter, a character like tent!Merlinus is, if anything, a handicap. Convoy units taking up a party space a la FE6 is just dumb, also.
I have to say, I just don't really care very much about the convoy beyond having one.