In terms of talents and legendaries I would essentially use the same ones for both Ret and Holy for all three of those fights:

For ret, cloak/talent ring will be used for every fight regardless of number of enemies. Its possible I would take divine hammer over blade of wrath for Eonar if I was assigned to solo certain felhound packs but otherwise would use standard talents (Final Verdict, Greater Judgement, Blade of Wrath, Crusade) due to how well they perform in any situation.

For holy I would wear Ilterendi/Prydaz on almost every fights due to how effective they are as well as the fact that my non-legendary neck options are much lower item level than my other slots. I would use standard talents for all three of those bosses namely Bestow Faith, Rule of Law, Aura of Sacrifice, Holy Avenger, Judgement of Light and Beacon of Faith.

In terms of playstyle/maximizing performance:

On Felhounds as ret, I may swap out Seeping Scourgewing for arcano or another trinket if we are stacking them the entire time. Otherwise the fight plays out like a pure ST fight for ret with very basic stack/spread mechanics on a timer. For holy I just make sure to direct heal tanks more often than normal due to how much tank damage there is on the fight. I generally try to position myself in between the dogs while the siphon isn't occurring in order to ensure both tanks are receiving beacon heals, but will just swap a beacon to myself if one tank is out of range and I am unable to reach him safely. Most of the damage comes from siphons so I will try to have a throughput cooldown for each one.

On Eonar as ret, I may swap out Seeping Scourgewing for an AoE trinket or arcano but the gain is quite minimal. Throwing out off heals with Word of Glory or flash of light while healers are a little spread out can save people as well. As holy I generally am assigned to follow one of the two tanks for the majority of the fight and priotitize staying with them. People are often spread out doing different things at certain points of the fight so I make sure to keep track of people dipping low and fly over to them if needed.

On Kingaroth as ret, depending on the dps of my group that goes to kill the assigned robot, I may pop wings a little early on the boss to build stacks of crusade before we transition to maximize dps. Ideally my wings drop off just as the increased damage debuff falls off. We give the boss the increased damage buff so the last set of robots is actually quite healing intensive and a Word of Glory with 15 stacks of crusade can actually save lives despite it feeling awful to use holy power on healing with wings up. As holy I save minor throughput CDs for the adds that spawn, and will generally us my raid CD once we have all grouped together for the very last add.