Oooh the item crunch, one of the most personal and hardest areas to optimize.
* Do
not fall into the trap of thinking 760,000gp is a lot and spring for frivolous items. Like you initially only listed two Enhancement items and were instantly talked into spending a whopping 128,000 more gold on a Belt of Magnificence you honestly don't need and can optimize better bonuses for less gold if you did. On the same token, Eye of Winking is crap for 120,000gp. If you're worried about your Mind Blank being dispelled consider a Legacy Item, Craft Contingent, Contingency, or Spellblade (I think that's their name) Armor/Shield Spikes, or since it cost them their Action to Dispel it just recast it next turn because if all they do is try to remove your buffs you're still effectively casting spells that deny them useful actions as your party beats on them.
* If your DM is willing, claim you met a True Dragon and he made you a Runestaff, place stuff like Consumptive Field on it.
* If you have Persist Spell, a Minor Schema of Metamagic Item costs 6,400gp and turns any Spell Trigger item into a buff.
* Don't be afraid of limited resources. A Staff can give you an extra five Spells to access on the fly (or persist) and those Spells are cheaper than a Wand to access (3rd~5th spells are up to 50% off). Versatility is key.
* Necromantic Empowerment loaded in to one of those gives you a +8 Enhancement Bonus to Int, and a bunch of other bonuses, which when combined with Persist (and possibly extend because why not?) it'll give you even more Bonus Spells per day than your item you have now and it's cheaper.
* Ask your DM about UMDing the Horseshoes of Flame, they take up your Hand/Boot slot but provide a +10 unnamed bonus to Intelligence. Not all shoes are nailed on and honestly riveting them to a pair of leather gloves to don would work anyway.
* You can buy multiple items, like you can buy four Belts of Battle and blow their charges over four different Encounters.
* Don't overlook the small stuff. Marbles were just effective as being better than Grease at level twenty as they were at level one, and oddly the Spell is still just as effective against Medium Creatures too.
* Two quick threads to consider,
Class Boosters &
Bunkos.