Hi Miles. If your card came from another postcard collector, then the sleeve you received it in is almost certainly acid free, so I doubt you have a problem. You are right, 300 really isn't so many and, ofcourse, each of the messages is unique.
I was lucky to see the cards, still in their album, because my friends at Godalming Museum were going to try and acquire them, to bring them back to his home town. I visited Christie's with the Curator of the Museum when she went to view the collection. I did not bid at the sale because I didn't want to bid against the museum and force the price up, but sadly they did not win the collection anyway. Naturally, no auction house will ever reveal the identity of a vendor, but through research over the years, I have a pretty good idea of the history of the cards.
I know several other Phillips collectors but I doubt that our combined collections add up to 300 so I'm sure there are still plenty of cards out there for you to find!
Finally, yes, there is a card on which Jack mentions going on Titanic. He posted it from Belfast and it had a picture of Titanic under construction. He seemed rather disappointed that his secret was out of the bag because he grumbles to Elsie "I suppose Ethel's already told you I'm going on Titanic....." You can see a picture of this card in the book 'Titanic- Fortune&Fate'