I'm also a Soraka main. I have around 1 million points on Soraka and have been playing since Season 5.
Yuumi is a lot like old Soraka in the sense that she doesn't have much interaction, which is funny, because the whole reason Soraka was reworked was because they wanted her to be more than just a healbot. That was her reputation, and it's one that persists to this day. Now she does way more, but then they create Yuumi who has almost zero interaction. Good ol' Riot for you.
Soraka was indeed reworked because that kind of enchanter is frustrating to play against. Typically enchanters need to be vulnerable if they're going to provide that much utility, and part of the skill is learning how to position yourself and learning the rhythm of trades.
Yuumi does not heal more than Soraka though. Nobody does, and if they do, then you were playing Soraka wrong. Yuumi may be able to burst heal more than one of Soraka's W, especially early game, but she cannot spam heal like Soraka can.
What I will say is Soraka very much isn't a dedicated healer early game, and that's a misconception that many people have.
A lot of ADCs think Soraka stands back and heals, and does nothing else, maybe throwing out a Q here and there. The way Soraka is meant to be played is much closer to a lane bully early game. You want to be constantly harassing your opponent, and if you're not careful, Soraka can bully or kill you easily if she isn't contested.
It doesn't feel like that though because she doesn't do the insane burst damage of a mage, so what enemy ADCs will do is sacrifice their damage and buy Executioner's Calling on the first back, and proceed to lose harder because they're losing trades to her harass and not her heals. Soraka transitions into more of a healer later in the game, and that's when you want anti-healing because her heals are crazy.
I would say what is most deadly about Soraka, especially in this one-shot meta, is not her heals. It's actually her silence/root. Equinox can turn the tides of fights and trades, and completely screw over assassins if it is used properly. You can even cancel ults with it. There are not many AoE silence+roots in the game, and hers is instant.
I wouldn't personally change Soraka's heals. What I would love to see is the missile time on her Q reduced. You really do need to stay interactive in this new meta as an enchanter, or you get steamrolled.
The only reason Yuumi was fine was because you had to kill her ADC to get to her, which when facing a problematic enchanter, you want to kill the enchanter first. That's why she started to develop such a high ban rate before she was hit with a series of nerfs. Riot was trying to seriously cut into her numbers to try to balance out her safety.
makes it so soraka heals better than yuumi.