Magical secrets come at 10th level bard, If I´m not wrong. You do get different spells because you multiclass with a sorcerer, sure, but you have to pick spells from the mage pool, not cleric, druid or warlock spells that are very useful for a bard that is not a pure caster, like sword and valor bards, but it´s a very strong build to multiclass with a sorcerer, yes.
I think is level 6.
And the main advantage is not the sorcerer spells pool but to be able to cast twice on the same round, or apply disadvantage to saving throws through metamagic.
