I think rogue's slippery mind is too late. Actually any ability that gives proficiency that late is not the most elegant thing...
It gives you an instant bonus of +5 to will saves.
And it makes you think twice about taking resilient earlier.
In my opinion giving advantage on will saves at that level would have been the better choice. And mathematically it would not be a lot worse.
Monk gets prof in all saves later on. I actually like it.