That said, you can actually have higher Spell DC's as follows: (my set up, yours may vary
)
For filigrees (+9 INT above) you can instead have the Eye of the Beholder 4 piece set and Otto's 4 piece set for +4 to DC's and +4 to INT (Eye of the Beholder/Inevitable Grave +2 INT filigree, Eye of the Beholder +1 INT filigree and Otto's +1 INT filigree) for a net difference of +2 DC's and -5 INT over the MAX INT setup.
That's 124 INT and +2 DC's to all spells over the MAX INT set up
Epic Feats in the MAX INT setup is 4 INT;
Instead use one of these feats for the Embolden Spell feat for +2 to DC's (a no brainer as +2 DC's is the same as +4 INT). This is a net difference of +2 DC's and -1 INT over the MAX INT setup.
That plus the above filigree change puts you at 123 INT and +4 DC's to all spells over the MAX INT setup.
Enhancements: 15 INT in the MAX INT setup (+8 INT from Palemaster, +4 INT from Harper, +2 INT from Deep Gnome, +1 from Archmage)
Instead, spend 12 points in Harper for +2 INT instead of +4 INT as in the MAX INT setup, and spend those points in the Deep Gnome and Archmage trees to gain: +3 DC's to Illusion DC's in Deep Gnome, +3 spell pen in Archmage, +1 Illusion DC in Archmage, PK SLA in Archmage, Displacement SLA in Archmage, and another +1 INT in Archmage. This nets +4 Illusion DC's and -1 INT from the MAX INT setup.
This plus the aforementioned changes puts you at 122 INT with a total net change of +4 DC's to all spells plus an additional +4 DC's to Illusion DC's over the MAX INT setup. (DC bonuses to a secondary school (instead of Illusion) is possible depending on your setup).
Max Sustainable INT while also having MAX DC's: 122