It honestly doesn't matter how much you buff her now, you released a dual-lane champion that no one wants to lane with because she has no peel and no lane presence. Her first impression is completely ruined and it's going to take months for support players to be able to pick her without having own-team bans if we try to hover her because you screwed her launch up so fcking badly. As a support main who's been desperately waiting for a fun new champion to play this is incredibly depressing.
Next time you put out a support champ maybe give them more than a week on PBE. Her launch was incredibly rushed and there were warning signs everywhere that she was weak, but no chance was given to fix her until she hit live servers after which you had to buff *literally everything* on her and it only made her... "okay"-ish, maybe, into SOME lanes, and certainly not most of them, because again:
NO LANE PRESENCE
Enchanter's fantasy is to protect your ADC and keep them alive and fighting. Yuumi can't. All she can do is cast two heals and then die with them. It's the most depressing ANTI-fantasy I've ever seen. Literally the exact opposite of her class fantasy.
Make her ult stun on 3 hits, give her 50 range to match EVERY OTHER RANGED ENCHANTER, and maybe people will want to lane with her. Maybe. In three months, after the HORRIBLE toxic launch fades from memory.