While I'm flattered that you think I know anything whatsoever about that topic, I'm afraid that's not the case. All I'm good for is bits and pieces I nick from Metzger or the Alands. As you point out, the NT, much less the entire Bible, wasn't circulated as a whole for centuries. My somewhat shaky and clouded recollection is that for many years, it was rare for churches to have copies of all four gospels, due in part to their length. Paul's letters were shorter, and I believe were bound together sooner than the rest, which made them a little more common. That's about all I can offer on the topic from memory, and even some of that might be inaccurate.