'is it really that bad if I just play exclusively support' - I mean, yes and no. Generally speaking, most heal/shield supports low Diamond and under don't have a proper understanding of roaming, harrass, or trading, they're just purely reliant on their kits to carry mid/lategame teamfights and their adc's want to pull their hair out in lane. It's really tilting.

There isn't really such a thing as a 'passive' or 'aggressive' playstyle - there's just correct and incorrect times to do things. If you play Janna for example and never autoattack, any reasonable adc is tilting. Just because you CAN win games from random mid/late teamfights where you shield/heal doesn't mean that you're playing correctly (esp. in lane). If they're hitting your adc, and you aren't hitting them, you're wasting free damage. Early game 4-5 autos is half an adc's hp.

'I think in the long run will benefit my skill and my knowledge' - one champion can help, but what you're looking for / your attitude towards learning the game is going to hold you back.

tl;dr: Janna/Soraka are the champs you are looking for. The way you play it will make your adc want to pull their hair out, but you can hit D5 with it if that's your goal. It won't necessarily hold you or your knowledge back, but the way you intend on playing them will. Hope that answers the question.