Diamond ADC/Support main here. I am also 21. As well as on a collegiate team for Eastern Michigan University.
Looking at your games recently, you rush mobi and captain enchantment. I don't agree with rushing the captain enchantment. It just delays your item path too much. 450G is a lot. Which helps with being able to take more damage or having more health. You already give enough movement speed off with W on Janna.
For runes, I am totally up for testing stuff out and that, but because of how high damage ADCs are now as well as top/jungle, you do need more armor than CD. So the usual runes would be AP quints, armor seals, MR glyphs with 4 mana/5 glyphs, then you can either have AD marks and make yourself more able to trade with people, or you can go HP or Armor for more tankyness. I know you shouldn't be tanky, but it allows you to take more damage. Which is what is needed for supports as of right now that I'm seeing.
I am not opposed to CD runes, but it just seems bad because you can't use that CD if you're already dead because you have no defenses. You know what I mean?
Another suggestion. I know IEM is being played on 5.21, but after the matches, you should be able to go on lolesports.com and look at the matches and what the supports are running for runes. It shouldn't be that much different from what they are running right now. As well as you can look up someone like Aphromoo and copy his rune page(s).
I hope this helps. If you would like more help or have any other questions, feel free to add me. I have helped a lot of people. As well as sorry for incorrect sentences lol. Long day with 5 hours of a long training session with collegiate team. But I do like helping people out. :)