AnotherGuy
Hero
It can easily be solved with giving a monster are class feature or feat (not the ability bonuses) for every 4-5 HD with Resilient being one of them.Bounded accuracy actually made it worse because it justifies not increasing a monster's bonus from being high level due to the bounds of the bonuses but it justifies the casters DC increasing by level which creates the imbalance of the caster having a high DC but the monster having a low bonus.
Now if you are willing to say every monster of CR8 or greater it has Proficiency or Expertise in every saving throw and has a 15 of better in ever ability score then fine There's no problem.
So
Accentuate the tankiness of your Hill Giant with 12 HD by giving him 2nd Wind, Savage Attacker and Resilient.
Heighten the fluidity of your Water Elemental with 12 HD by giving it Mobility, Resilient and Cunning Action (Dash and Disengage)
..etc