What's the most efficient way to acquire Dark Oak Saplings?

Honestly, dark oaks require time to farm, and there's not really that much you can do to speed up the process. But you CAN do is:

• Come prepared with all necessary axes.

• Plant as many trees as you can at a time.

• Aqueducts of water gathering into a single place surrounding little islands of the trees could speed up sapling gathering. Also enables you to be afk while the remainder of saplings fall, without worrying about them despawning while you're afk (make sure to have hopper at end of water). The islands would be 2x2, with a single block on the side to jump to if the tree has grown.

• I'm not a tree expert, but I know there are ways you can modify the height at which trees will grow. Try to make adjacent trees grow at different levels, to optimize the amount of leaves spawned.

• Yes, diamond efficiency V would help. :)

• If you're interested in AFKing it, don't worry to destroy the leaves manually.

• At night you need torches to grow saplings. Put 1 (minimum) on the jump-to block to maximize efficency.

These are pretty much all the ways I can possibly think of, and am 95% there's not much else you can do. Do all of this, and you should increase your efficiency by at least 2%! :P

You can, of course, just find a big dark oak forest and start chopping to your heart's content ("acquire Dark Oak Saplings") but I infer that you mean farming. But if you don't mind a big plain where there used to be forest, this is quite efficient (especially when you come fully prepared, especially with torches)

You can also use bone meal to remove the need to AFK the saplings' growth, but you'd still have to AFK the sapling drops from the leaves.