yummi gonna be like old soraka, it doesnt really matter what you do since she can just heal her adc, a passive playing adc wont in danger any poke gets negated by yummi, if they nerf her healing and focus on her poke the fact that she offers poor counterplay due her W still leave issues, she is like sona except that if you catch sona she explode into pieces with her poor defense. yummi might never be balance in pro play and will be one of those champions that are too strong for pro play but to terrible in solo queue riot will rather make ppl forget about them like kalista and azir
offered solutions:
make her W have a real colddown early on.
leave her heals strong but make it so she can only gain more than one charge while not attach.
this means she wont be attach at all times early on now to fix her late game we need more reasons for yummi to not be attach, her passive is a great idea, the mana recover was only relevant early honestly, we need something yummi cant afford not to do late game to force her out of the attachment kinda how soraka needs to land her Q to recover her own HP oppose to old soraka that would just spam W as needed, her ult is not bad at all but the fact you cant stop her and she isnt vulnerable while using it is a bit too much