In my attempts to hack away at 5e, I have tended to put "you are good at saves" abilities at level 11 or higher in martial classes.
This (a) prevents double-dipping via multiclassing, and (b) helps populate the back-10, and (c) lines up with when spellcasters get their T3/4 spells.
The Paladin existing level 6 ability is the exception to this.