Building a Sacred Stones team

Getting a "pity" Myrrh instead of Hector made me quite salty at the gacha so I've just decided to put her to good and flavorful use.

I'm currently planning on a Sacred Stones team with the following units:

Legendary Ephraim(-HP, +DEF, +1)

• A - Death Blow
• B - various/Renewal/Solar Brace
• C - various
• Special - Aether/Bonfire/Moonbow/Glimmer
• Seal - Heavy Blade 3
• Support - Reposition

Lute(-HP, +RES)

• A - Fury? or maybe Fortress RES would be better
• B - Desperation
• C - ATK Ploy
• Special - Iceberg/Glacies/Moonbow
• Seal - DEF Ploy
• Support - Reposition/Reciprocal Aid/Ardent Sacrifice

With these two alongside Myrrh(-RES, +DEF, no build yet and open to suggestions) as my core team, who would be a sufficient fourth member? Currently, I am considering a red which would mean either Eirika Sword(as I do not have the mage) or Lyon. Also, what build should I aim for on putting Myrrh and on the fourth member? I was hoping to put tactics on my team save for ATK Tactics as I was not able to pull Legault.

Any opinion on the matter is welcome. Thank you!

EDIT: Just remembered other choices for red - Set(as suggested below) and Joshua.