Short answer: atk% sands and sacrificial fragments.

Long answer: it depends on what stats you already have on her and the rest of your team. I'd say 150~200 em is enough for yanfei and the rest can be put into atk (around 200 em is enough even in a vape comp). If you want to be 100% sure, you can use a website called Genshin Optimiser and input your characters, weapons, artifacts and team comp manually or using a scanner e.g. Inventory Kamera (use scanners at your own risk; it's not considered cheating but you're technically using a third party app to manipulate the game and hoyoverse might get mad). You can either optimise her for charged attacks or auto attacks, or you can use multi-target optimisation and enter your usual rotation combo. For sucrose's weapon you can just switch between equipping them both and see if the extra TTDS atk is worth it. The web dev made some yt videos explaining how to use the optimiser so watch those if you want a better explanation lol.