Nice to see a bard guide. I think I would probably make the DD part of it a separate guide if I were making it since it's gonna get really long otherwise.
There's some things I don't agree with or think should be added.
Merits should be 5/5 Minuet 5/5 Madrigal, 5/5 Troubadour and 5/5 nightingale eventually. Finale and lullaby recast merits are fine to hold you over until you have enough equipment to get song recast time -18.
5/5 Troubadour has unerring song accuracy effect. This is like elemental seal for songs except it doesn't wear off after 1 cast. It is usually not necessary to sub blm for Elemental Seal. I thought it was funny this wasn't mentioned considering the guide name.
WHM is my sub of choice for haste, Cure IV, Stona and other -na spells. Maybe if you are just starting out RDM sub is useful if your song spellcasting set isn't done. I can't comment on SCH since I've never used it. I have subbed THF for TH2 as well.
March increases attack speed rather than reduces attack delay in ffxi terminology. Lowering attack delay reduces tp per hit. (see blitzer's roll). Similarly elegy reduces attack speed rather than increases attack delay. Isn't hugely important though.
Marcato does effect Elegy, Lullaby, Requiem, and Threnody I believe.
Elegy (potency, although elegy spells cap at 50% slow)
Lullaby (acccuracy I think)
Requiem (potency)
Threnody (potency)
It's probably not worth using with a debuff over a buff though. Perhaps with Ltng. Threnody for when stunning certain things.
I think I would still buff in 5/5 empyrean armor for set bonus except for when I'm using SV/Marcato or cycling parties. This is probably down to personal preference though.
For debuffs I would aim for a set that lowers recast for debuffs to 4 seconds. Something like this:
This set can be improved still.
Magian Magic Accuracy Staves (or Carnwenhan)
Hyaline Hat, Shedir Turban or Marduk Tiara+1 if you can make up -recast in other slots
Could replace AF3+2 legs with Marduk legs+1 too down to personal preference. (song recast-6 vs duration+10%, depends on song and situation)
Some proper M.acc feet too. I personally don't bother since I like to be in my movement speed feet when I finish casting.
4 second recast Horde Lullaby and Horde Lullaby II are really nice. Daurdabla instead of G.horn for Horde Lullaby and Horde Lullaby II for larger AoE.
For Voidwatch I'd recommend one of your Atmacites being
Atmacite of Exhortation for Sphere: regain+2. You can stand with your melees and use Fanatics/Fools to avoid damage and back off if things get nasty. It's nice for tp ticking up when
that person doesn't clear the box.
Lullaby +1 gives +10% duration to lullaby. +3 seconds duration to tier 1 lullabies and +6 seconds to tier2 lullabies.
SV doubles duration of Mazurka/Scherzo rather than potency.
Bard's relic horn, empyrean harp and mythic dagger are all really nice and unlike other jobs they all work together so it isn't a choice of which to equip. The best order to make them in my opinion is
Daurdabla 90 -> Daurdabla 99 > Gjallarhorn 95 > Gjallarhorn 99 > Carnwenhan.
There are +3 instruments for most songs meaning the 95 Gjallarhorn is only equal to those and better after doing the trial for 99. If you do make them in that order you will want to try and get the Langeleik for march+3 from Celeano to use until you have your Gjallarhorn.
When using 3 or 4 songs you need to watch out for dispel moves or melee running away and missing songs as you will have to reapply with Daurdabla.
Generally March x2 are the best songs to use. If all your melee have embrava and haste they are capped magic haste(for now) so your march won't be adding anything.
Songs don't always overlap as desired.
For example: Say you have used Troubadour March March Minuet Minuet on your DD and want to ballad yourself but your Troubadour wears off and you try to ballad x3, You will probably end up with Ballad March Minuet Minuet due to the song with lowest duration getting replaced.
Different people have different styles to playing bard so I'm not necessarily correct in everything.
I have probably missed some stuff.