Yep, that's more or less the intention of my suggestion. Maybe add in the recipe, but remove the random drops out of the tree, that way, in order to get saplings, you would have to already have moved farther down in the Thaumcraft research tree, and harvest the tree very carefully in order to maximize the leaf blocks you get with the Shears, to get more saplings with the recipe later. This would make Silverwood Trees a renewable resource, but i don't think it's overpowered, especially if the recipe is further down the Thaumcraft research tree, meaning that before you can get access to what's basically Grow Your Own Pure Node (tm), you will need to have dealt with a lot of flux in the meantime... I'd like that better than my previous suggestion, actually. A guaranteed 100% way of getting a sapling, but removing the random drop aspect of it. I'd personally put it at the same tech level as Crystal Clusters, but before Crystal Core.
The way it is right now, sometimes you get 2 or even 3 saplings out of a tree, several times in a row, and end up with a lot of saplings, and sometimes you harvest 3 or 4 trees and get no saplings. Making the process of getting a Sapling more involved with Thaumcraft research is nice, and being able to grow a Pure Node out of thin air i think is a pretty good reward for getting to the mid-late game of Thaumcraft.