Sucrose's Ascension 4 Talent boosts the entire team's Elemental Mastery by 20% of Sucrose's total Elemental Mastery, which is why you want to stack as much Elemental Mastery as possible in her artifacts. As a support, most of Sucrose's value lies on her Ascension 1 and 4 talents, hence leveling her other talents (Normal Attack, Skill, Burst) is not required.
When utilized as a (4) artifact set holder, make sure you don't immediately swap away after casting Skill/Burst, you need to be on-field when the Swirl reaction happens for you to trigger the 4-set effect.
Regarding Weapon Choices:
There are some team compositions in which Sucrose's Elemental Mastery is not a priority, such as Freeze teams or an Anemo battery for Xiao. In such situations it's better for Sucrose to use [R5] .
Thrilling Tales of Dragon Slayers: This weapon does not provide any Elemental Mastery, however it overtakes the other weapons on the list when it comes to buffing your team. It offers a 48% ATK boost to the character you swap to, which is equivalent to 1 main stat. This buffs your DPS more compared to , which gives ~44 EM, equivalent to only 2 substats.
Hakushin Ring: This weapon is a good option for teams where Sucrose can swirl Electro to trigger its damage-boosting passive. In Electro-Charged teams, she can buff Hydro DMG% as well.
Favonius Codex: This weapon can be useful if you value the extra energy generation from the passive. You may need to get a few CRIT Rate substats to trigger the passive reliably.