Fully supported tours don't really appeal to me. To some extent it seems that they defeat a couple of the purposes for touring. You would lose the feeling of crossing the distance without relying on cars. You would lose the feeling of independence. And you would lose some of the opportunities to interact with the local folks. Then there is the cost...

You don't give any clues of whether you prefer the US, Europe, or somewhere else. I will limit my comments to the US since that is where I have toured.

The Trans America is a great route and can be easily done is your time frame. If you want to get a feel for what the rest of the US is like it is a great experience.

The Pacific Coast is beautiful and very well suited to cycling. Camping is readily available and cheap.

The new AC Sierra Cascades route (my personal choice for this summer) promises to be beautiful and challenging.

If I had more time, as you apparently do, I would ride down the Pacific Coast and back up through the mountains on the Sierra Cascades route. That way you would hit much of the best scenery in the US. You would also have a chance to become a bit more road hardened for the mountains before you get to the Sierras.

What ever you choose, have a great trip.