Sayu does more damage with Elemental Mastery builds and heals more with ATK hybrid builds, Healing Bonus circlet's can be used for better heals while maintaining respectable damage. Note that Sayu wants a large amount of Energy Recharge when used as the only Anemo slot on the team.
Use hold Skill then immediately press it to generate more particles.
Regarding Weapon Choices:
Wolf's Gravestone: This is a universal claymore that is capable of buffing your teammates along with providing more healing in Sayu's case. It is best used once you achieve your Energy Recharge threshold.
Forest Regalia: When used in its niche with Aggravate teams it can outperform . Outside of that its just an Energy Recharge stat stick.
Regarding Artifact Choices:
Sets
Viridescent Venerer (4): The bread and butter artifact set for Anemo units, this set provides the ability to shred the enemy's elemental resistance while also buffing Sayu's Swirl damage output.
Ocean-Hued Clam (4): This artifact set trades Sayu's utility in favor of stronger healing and some extra damage. It is generally not recommended to farm this set for Sayu specifically.
Substats
CRIT Rate: Only recommended if using a Favonius weapon.