I am a low elo noob, but I play a lot of yuumi and i think this build just feels overall good on her, I feel consistently impactful, even if adc is lacking. After playing the typical max shield > w > q and pure healing/shielding build, you just feel too useless if ur adc is smurfing on them. So i started experimenting and imo this has been feeling so good to me I thought I would try making a guide to share, even though im silver-plat elo player. But maybe some other people would enjoy playstle :) Yuumi is so good and fun to me, I think a lot of people dont play well enough, mainly i like this build to be more aggressive and also hopping off to fit in auto attacks when its safe, especailly early in lane, is HUGE, goign to add a decent amount of dmg with ur auto's early, after u shield and q, hop off and help secure a kill with autos, if ur sitting on ur support with q and e on cd, the only thing u giving them is a tiny amount of on hit healing, which does help some but ur autos will hit harder then the on hit healing by a lot so its def better to be auto'ing when in scraps early. Other than that gotta land those q's, get us to aiming it and moving it to land more consistently, cause when u land a few back to back q's u can really push people out of lane or make them tower hug, making life way easier for ur adc to farm or go for kills.
But i have fun with this build and feel consistently impactful, but of course this is my personal take, hopefully u enjoy, but can always try different runes/items, whatever works best for you :)

Situational items, all good but can pick what u think will help u most

My Personal Special Mentions:
Dawncore, giving healing and shielding and ap, i feel like thats practically core after moonstone to me at least to get her ult healing really up to for keeping team alive to me, lots of good stats.
I think Knight's Vow is hugely underrated, even with no ap/healing, 12% perm damage reduction is pretty strong, it absorbs big damage every time i get, i like to get this last or 2nd to last but it kinda depends on how game going, but typically i might take if my carry getting exploded too fast, and i only get 1 shield and 1 or 2 ticks of ult before they die, basically if they dying too quickly for u to really get ur heals/shields on them well, is when i strongly consider fitting knight's vow into build.
I also think Redemption is super OP IF u can land the active consistently, when i focus more on healing yuumi my favorite was redemption -> moonstone, but especially when everyone in game is dashing all over the place or if u just miss the active on team, it feels bad, but i think redemption is possibly the best item if land active, but i miss it too often so i kinda gave up XD maybe if there a jarvan/camille some kind of lockdown where it looks like teamfights will be clumped in obvious spots i might take.
Mikeal's Blessing is a good one of course too, if they have ashe/leona or som kind of cc that just makes it too hard for ur carry to play, and ur adc/carrry doesn't have cleanse/spellshield, I would def try to get this, maybe even before moonstone, especially those far ashe ults that stun forever, can be a real life saver.
I think rest are mostly straight forward, and when im trying full on healing i use to go Echoes of Helia, even though i always that it felt bad since ur not a champ that auto's typically to really gain charges fast, since its just with ur q's and ult that can proc it, but i guess especailly since ur ult constantly hitting and healing it seems to actually perform good looking at the healing it does when mouseover item in game, when i was pure healing yuumi i started to go echoes -> moonstone when i gave up on redemption after missing lots XD and it seemed to be solid healing, plus the rest of stats good its like another moonstone almost. but starting with Luden's style I would probably never buy Echo's of Helia, but hey , maybe i should test out sometime cause it is some dmg too. idk, but i prioritize moonstone / Dawcore I think too much personally.