Ultimate Master Spell Thief Sublime chord!
Spellthief 1/ Bard1/Wizard 3/Unseen Seer 3/UM 1/Sublime Chord 2/UM+9
Decent caster, can handle blasting if you pick the spells, has decent amount of skill points with a decent Int. Needs Cha anyway so Diplomancy is up for grabs, also gets some bard spells so he's set for that task.
And his caster level is around 50.
You should check out the Trickster ACF for Spellthief from Dragon Magazine 353 pg 85.
While you still need bardic music for Sublime Chord, the ACF could (depending on how you rule it)... stack with your bard levels to determine bardic spell progression and could combine the spellthief/bard class spell lists.