I've told you why it is bad. His kit is powerful as it is. He's got an impressive heal when stacked up to 5, he's got a %HP shield (which caps at 12% if I'm not mistaken) he's got an AOE stun that can't be interrupted once cast, and he's got an AOE invincibility. He's got a ton of power in his kit already. I've mentioned this above.
Having the capacity to double all of what he's got would overpower him. He would heal virtually all the damage ever dealt to him in huge bursts, but that's factoring in if he even takes damage at all with a shield that blocks up to 24% of his max HP or a 3 second invincibility, and a stun that would simply stun enemies if he moved past them with the telegraph out. Your recommendations would make him so OP, he would need to be nerfed into oblivion.
In fact, the only thing keeping Taric from going into a solo lane effectively isn't in his kit - it's just sheer numbers. He doesn't have the mana to functionally manage a solo lane, and when it comes to sturdiness, he doesn't have the base HP/AR/MR to withstand all of the higher early and sustained damage that exists in other lanes, he needs a few items built to really be a threat.
As a support, he doesn't need to have anything done to his kit. His kit interacts very well with his allies, and he's one of the few champions in the game with the capacity to reliably heal and shield his allies, while also peel for them with creative angling. He's a very fun support, and not at all a heal bot like Soraka or a shield bot like Janna.