Yun Jin build recommendations
Yun Jin is a Support character for Normal Attack focused characters who can deal off-field damage via her Elemental Burst. Yun Jin's Elemental Burst grants nearby party members 30 stacks of additional flat damage bonus on Normal Attacks which scales with Yun Jin's current DEF. This means that external DEF buffs like from Gorou's Elemental Skill and/or Burst doesn't provide much to Yun Jin since it only buffs active characters and not off-field ones.
Regarding Weapon Choices
With this weapon, try to aim for some Crit Rate stats to trigger the weapon passive reliably.
Although the EM mainstat isn't useful for Yun Jin, the passive decreases her Energy Recharge requirements by alot, especially at higher refinements. It can reduce her ER% reqs from around ~20% at R1 to ~70% at R5.*Note that she must cast Elemental Burst first before Skill, as the passive drains 3 energy initially when Skill is cast. This weapon can be left at level 1 while still getting full effect from the passive.
Regarding Artifact Main Stats and Substats Priority:
Crit Rate should only be considered when Yun Jin is equpping Favonius Lance to reliably proc the passive.
Artifact stats priority
def% er def crit rate