Change "save DC 14 + spell level" in spells prepared to "save DC 14 + spell level or 15 + spell level for illusion spells)"
As for CR, we can look at it one of two ways:
1.) If it were a 20th-level human wizard, it would be CR 20, right? At least CR 17 for the minimum caster level to cast 9th-level spells.
2.) If we were to just determine the CR for the tybor without its spellcasting ability, and then add wizard levels as non-associated class levels, what would it give us?
3.) Find another critter with 20th-level spellcasting ability that isn't CR 20+.