Yun Jin is a Support character for Normal Attack focused characters who can deal off-field damage via her Burst. Yun Jin's 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 Skill and/or Burst doesn't provide much to Yun Jin since it only buffs active characters and not off-field ones.
Viable Weapons with WIP Calculations:
Viable Artifact Set with WIP Calculations:
Regarding Weapon Choices:
Favonius Lance: Try to build some CRIT Rate if using this weapon to trigger the passive consistently.
Kitain Cross Spear: Although the Elemental Mastery Main Stat isn't useful for Yun Jin the passive decreases her Energy Recharge requirements by a lot, especially at higher refinements. Note that she must cast 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 the full effect from the passive.
Regarding Artifact Choices:
Main Stats
CRIT Rate: Only recommended if using a Favonius weapon.
Substats
CRIT Rate: Only recommended if using a Favonius weapon.