The highest possible charisma you can get (without using the succubus) at level 10, for a fletchling, is 18 +2 (racial) +2 (level bumps at 4 and 8) + 6 (headband) +5 (wishes), or 33. Obviously, such a scenario involves gold far in excess of what is appropriate for a 10th level character, but perhaps the campaign is rather "Monty Haul".
Assuming you have spell focus and greater spell focus in transmutation, that gives you a baleful polymorph DC of 28, which is certainly respectable, though a far cry from 35.
A rod of maximize has no effect on baleful polymorph - I think possibly you've misunderstood that feat? Also, you wouldn't benefit from the persistent spell feat in this case, because you can't cast 6th level spells (the level of persistent baleful polymorph). I suppose you could use a rod (as a full-round action, of course) or a staff of the master.
You could become a vampire for an additional +4 charisma bonus, pushing your DC to 30, and of course your eidolon could intimidate an opponent to impose the shaken condition so they have a -2 to the save.
Word of advice. As a summoner, Charisma is nice, but it's not that important. Once you have enough for your spells and maybe a bit more, constitution tends to be more important in order to have more hit points to funnel your eidolon's way in an emergency. Not worth going into debt to a succubus over. That's more a Sorcerer option.