And that is the source of the problem and the entire point of the thread because...
Your boss monster is now only making a save vs a control spell 10% of the time.
This is why this thread exists
Boss monsters rarely are at 0 when you're using CR 20+ monsters.
The CR 10 Aboleth has +5, -1, +6, +8, +6, +4. The -1 is to damage dex spells. That's just CR 10
The CR 16 Planetar is at +7, +5, +12, +4, +11, +12
The CR 17 Death Knight is at +5, +6, +5, +1, +9, +10
The CR 19 Balor... +14, +2, +12, +5, +9, +12
The CR 23 Baphomet +10, +9, +15, +4, +14, +3
What boss monster is going to be facing a DC of 19 with a 0? And many will have magic resistance and legendary resistance.