Soraka never goes full tank, "literally" or otherwise.
Stop saying that. Whenever you see someone say "Soraka has gone full tank." You know they have absolutely no idea what they're talking about, have not played her or looked at her builds, and should not be speaking on her balance.
Going full defensive on Soraka is a bad idea no matter how you look at it. The reason Soraka's attempt to build health is to get Warmog's Heart for the HP Regen. passive, which is quite hard to get nowadays. A few patches ago most Sorakas got it by mid game, and that was broken. It's still a good item on Soraka, but that active isn't going to come into play until late game, which is hard enough to reach as a support. The moment Soraka enters a team fight, the passive is disabled. Similarly, it offers no MR or Armor for Soraka.
Her Q, the ability most Sorakas max first now, will not do very much damage or heal with a build consisting of only defensive items. Most importantly her heals will cost so much health compared to how little they heal, since healing scales off AP.
Repeatedly pressing W when you only heal a little bit is a good way to die quickly and help no one.
The more health you have = The more damage you do to yourself.
The more AP you have = The more healing done.
It's especially ineffective to build health early game since you need AP to be effective.
Lowering her base stats and heightening her AP scaling is the exact opposite thing that should happen, especially if your primary concern is Soraka late game.
Soraka is squishy at all points of the game, and considering that most Sorakas are reliant on landing Qs early game, you can find your opening really easily.
A typical Soraka build looks like this.

Building one or two defensive items does not make someone a tank, much less a full tank.
Complaining about how hard it is to kill Soraka when you catch her, and then complaining about her healing makes no sense.