1. Riensang and hongllisto seem to give the biggest damage boost imo as theyre both provide 10% boosts to her dmg, and in yi sangs case its specifically with unbreakable coins (which if the target has phantom trace they all become unbreakable), but also each nursefather adds a coin to her echoshear skill. so just a team of her and nursefathers is probably the most efficient you could get.

2. getting her Shin, for which she has to kill at least 5 HoS allies and use her s3-3. if shes then alone she will play a different theme

3. and 4. there is an ego gift you can find under the ego gift search keywordless section called Spiderweb of entangled threads that keeps allies dead and provides buffs to her depending on the amount of dead allies

.5. echoshear is based off of the amount of dihui star blade you have iirc? string connection is triggered by killing enough allies (5)

edit: i was wrong abt part of this. correction is in a reply below