Wizards who don't multiclass (there are dozens of us!... In all of Arelith's history...) can only use the +2 INT ioun stone, if they don't multiclass to grab UMD.
Using a +2 INT ioun stone is kind of unthinkable, because it means that you will be resting and losing spell slots. You effectively are just missing spell slots permanently. Or, instead, you plan your gear without ever getting to use an ioun stone.
Suggestion:
Literally just add caster classes to the Orange ioun stone's allowed classes. The way it's set up currently, a pure caster character will be forced to not use an ioun stone because the ioun stones for caster classes are all based on their casting stat, which means you rest and lose those spell slots because the ioun stone buff is dispelled on rest.