Bit late but I'd recommend
5/5 Madrigal
5/5 Minuet
5/5 Nightingale
5/5 Troubadour
An extra 5 accuracy to blade madrigal and an extra 5 accuracy to sword madrigal. Doubled when you Soul Voice.
Similarly with Minuet and extra 5 Attack per song. Potentially +25 attack if you cast 5 Minuets or 50 extra attack if you cast 5 Minuets with SV.
Nightingale goes hand in hand with troubadour. Gives 1min of instant cast songs if you 5/5 it. 1min20 with Bard's slippers+2, Bihu's Slippers or Bihu's Slippers +1 if 5/5. Completely negates the negative effects of Troubadour.
I can't stress how good troubadour is. Doubles song duration for your buff songs as standard. When you 5/5 it it basically guarantees your debuff songs will land if they can land and doubles the duration. Can't stick Elegy or Lullaby Troubadour it. Troubadour+Lullaby II+A full song duration set will sleep almost anything in the game that can be slept for over 5minutes and then you Sleep 2 or Repose over it. This gives you enough time to recover from a wipe, rebuff and continue. Lasts 1min. 1min20 with Bard's Justaucorps+2, Bihu's justaucorps or Bihu's justaucorps +1 if 5/5. Resist traits can still stop stuff from landing (resist-slow on elegy, resist-sleep on lullaby and you generally can't sleep delve bosses unless the aura is down). You'll be riding your nightingale and troubadour timers so the armor that augments it is well worth getting. 1min20secs to buff your parties and debuff your mobs). This lasts the whole 1min or 1min20seconds. It's not just a 1 cast thing like Elemental Seal. Combines great with marcato and spectacularly with SV for buffs.
Why not recast merits? Basically because you can cap recast through haste/march/equipment. Debuffs are generally 24 second recast the maximum you can lower the recast time to is around 5 seconds (4.8 seconds maybe?) Recast merits don't bypass this cap. there is an abundance of -song recast time equipment. Depending on your play style and what you do as events you might choose these but you're giving up something that can't be obtained in other ways for something that can. Minne generally isn't used much. Stacking defense and -attack does work but for the most part you only need enough to prevent being 1 shotted with a good healer and Scherzo covers single hit attacks.
As for Dirge and Sirvente while it would be nice to have a full spell list Nightingale and Troubadour 5/5 are just too good to give up and used every 10mins. Dirge and Sirvente would be situational at best and you would most of the times never give up a song slot for them.