Is Overheal a bit weak?

The main problem with Overheal is that at a low level, Triumph is just way way better. With more takedowns happening, you'll get more healing from Triumph than shield from Overheal. But at a high level of play with not many takedowns happening, Overheal is better. Buffing Overheal so that it's better at a lower level would just make it even more mandatory for high level play like it was a few months ago during the Overheal-Targon's meta. So I'd instead rework/replace Triumph and then see whether Overheal needs a buff.

It's something I really want to take on Taric since it could be really stat efficient on him, but it would have to trade places with PoM

Its just really situational while Triumph is more consistent.