Perfect for touring cars. Should be good for dragon flies with a TC.. A tube may Allow to get true macro but you might be manually focusing and this is very difficulty with such a lens but the results will be great. As you noticed 100mm is too short for most insects, the 300mm focal length allows you to be much further away to achieve the same magnification.
Just consider the price, the fact you will want a TC for wildlife and the lens really deserves a good tripod. However it is really the best lens out of all discussed. I didn't say much About it because I assumed it was out of budget or too heavy. If you get it you will love it most likely, if you don't it will because you find it too heavy, and if you then try the other options you will be disappointed by the slower aperture or softer images
.None of the other lenses will get you better images, you are not trading anything, it is outright a better lens.
Try it out before you buy if you can.