Not sure what the overall blocking is for the camera flyby. Is the camera starting from far away and zooming close to ground level or will it always be flying just above the treetops or always be close to the ground?
I ask because Gaea is a great piece of software, but it only truly shines when you need to see a vast landscape from thousands of feet in the air. That is where you can see the realistic effects of wind and water erosion on the landscape. If the camera is going to be no further away than the treetops, then you could get by using the landscape modifier in C4D and play with the scale a bit. In that type of scene, the focus should be on the vegetation, trees and rocks rather than the contours of the terrain as they may not be noticed that much (or at least only noticed as far as slight rises and dips in the surface).
If you need both in the shot (wide shot to a close up), then you have to worry about over-all polygon count and probably in need of a program that has been optimized for instancing. While no longer the popular program it once was, Vue has been improving and working on its integration with Redshift. From a spec perspective, Vue and PlantFactory will give you all that you need to get the job done and they offer a monthly subscription. So, apart from the learning curve, it may be the cheapest path (e.g., pass through the learning curve with the demo and buy a one-month subscription when you are ready to create something).
Other considerations are Forestor if the focus will be plants and trees. While they started development of RockGen for landscapes, not much came of it.
There is also Laubwerks which makes SurfaceSpread for distributing their plant catalogs across terrains. Personally, I find them rather pricey and the original developer of SurfaceSpread left Laurwerks and went to Insydium. But it is an option.
Now, all these options are about as expensive or more expensive than going back to World Creator which is now have a Black Friday sale for their professional license. It appears you are familiar with WorldCreator, so why not go back it?
I hope this helps.
Dave