Ooh, that’s an ambitious start. The reason I originally mentioned mountain-tops was because I figured that would at least save you some of the work; but I can understand how building your floating islands from scratch might be easier.
I don’t know if it’s possible to load a mod into the world once it’s started, but I’d strongly reccommend the Terrain Slab Colors Mod if you want to build your sky-islands from scratch. That would allow you to dress up your islands with “dirt” and “grass” (unfortunately it’s just a normal building slab which looks like natural terrain, you can’t use it like normal terrain e.g. for growing things) if you wanted to go for a more natural look. Of course, it also means more work when you go to build the islands…
If you prefer the idea of simple rocks floating in the sky, however, then you could simply use the normal stone slab colours – they’re not quite the same as the natural terrain ones, but it’s hard to tell the difference unless you have them right up against natural stone.
Either way, I’m keen to see what you come up with! I probably wouldn’t have time to watch a full stream of it, but I’ll definitely check back to see how the progress goes.
One tip though: mass-harvesting the whole forest is probably far more wood than you need right now, especially since you can farm trees later. However, since you have all that wood lying around, you might like to make it into furniture in order to level your carpenter up quickly. That will give you extra stuff to sell, building up your gold supplies for later; plus is means you can build the market stalls sooner.