I think it would be time to review Spell Resistance, with the current options this layer of defense does not scale well at all. The Monster CRs scale really steeply in order for it to take effect - often at all.
Exception is the Monk who is running in Grandmaster of Flowers, it doest stop a few spells. Although it really only matters if you are targeted with no save spells on a Monk.
I believe the bases could be scaled up further.
30 - lvl20
33 - lvl22
36 - lvl24
39 - lvl26
42 - lvl28
45 - lvl30
I do not think it would be horribly overpowered to get access to new sources:
Adding in a few stacking sources would help this defense stay viable for the rest of the game.
+ exceptional bonus +2
+ insightful 2-4-6
+ 1 luck bonus
very rarely +1-3 profane bonuses
Classes helped mostly: Pure Barbarians, Fighters, Rangers, Rogues.
The defender "themed" trees also need more loving vs spell attacks, they shouldnt be 95% about blocking physical damage where the real threat has always been offensive magic.