My advice would be: drop that idea, if that would help you in this case, which is doesn`t, so I don`t give you this advice
Extend Spell is good, since there are some other bard spells that might be interesting for this - Cat`s Grace, Bull`s Strength and similar spells.
Try to look at the summoned creatures and there special abilities - some of them can not only fight, but also cast healing spells, and this would give you some more options. (and there might be more)
Remember that summoned creatures can use flanking tactics and so on, so be sure you use this option, especially when you have a rogue.
Persistant spell won`t work on this, I believe. Isn`t it restricted to personal or touch spells only?
I don`t know about Prestige Classes for you, maybe the alienist is okay. The Gate Crusher might also fit - hey, you`re constantly summoning "things" from the planes...
A real weakness for you will be, that you will always use a spell slot of each level on a Summon Monster SPell, that might never be used when you reach higher levels. As compensation, you should consider metamagic feats like quicken spell or being hasted, so you can summon great groups of "monsters", so your enemy has a hard time to kill them fast. (But Fireball Spellcasters and Great Cleavers will be extremely effective against this tactic, so it might not be that useful... Besides the fact, that for spontenous caster, using Metamagic Feats is a Fullround Action, so Quicken won`t work, unless you take feats like Arcane Preparation, or you`re DM is willing to ignore this stupid rule...)
Mustrum Ridcully