Re-initializing a transactional replication wihch was initially synced from backup
So I had a similar situation.  I had a HUGE publication that took up to 36 hours to initialize with a snapshot.  When I had to move the subscriber to new hardware this just wasn't ideal.  So I used a backup to initialize the sub.  Of course I knew that we'd have to add articles eventually and I did what this article said to do in Option 1 (option 2 not available as there was no way I was going to be able to stop all activity on the subscriber short of a disaster).

Add Articles to Subscription initialized with backup

So I added a new publication to the same publisher database and then added the subscription to the subscriber.  Since the initial snapshot is very small (only 4 tables) it sych'd up very quickly.  Subsequent table adds to the publication are done on the new publication and snapshots generally are only of new tables, not entire publication.  Is working very well so far .