Ok, how about 2d6+CHA vs level-influenced number both on casting and on saving? For example an ogoing 3 damage. The caster rolls 2d6+CHA to beat base+target level/N, then the target takes his first 3 damage and tries to save against further effects rolling 2d6+CHA vs base+caster level/N. On the one hand the non-CHA character gets scaling defence by just levelling up, on the other, CHA doesn't get marginalized.
Let's see, worst case scenario, someone at level 20 has 1 CHA, thus their maximum roll is 13. If we don't include critical saving success, then the maximum difficulty should be 13. 7+Level/3 works, because 20/3 rounds down to 6. Also, if we have 7+ as the base save, then the probability of a CHA 1 character saving against various level effects goes like this:
Level 1-2 = 72,22%
Level 3-5 = 58,33%
Level 6-8 = 41,67%
Level 9-11 = 27,78%
Level 12-14 = 16,67%
Level 15-17 = 8,33%
Level 18-20 = 2,78%
By adding one point of CHA level 1-2 gets 83,33% chance and all others move down one slot. The result is that CHA based effects are easy to infilct, but difficult to maintain. It sorta balances out, though some CHA based abilities will need to be slightly modified to better fit the system. Also, this makes all ongoing abilities scale up with level advancement.
Incidentally, the increase in dispell level comes at the same rate as stat upgrades. So if you meet an opponent, escape, you both level up past a 3rd level and meet again, you might have the exact same probabilities. For CHA-based characters that means that, while fighting their own level enemies, they will always have 91,67%/97,22% (depending on class stats) chance to inflict the ability, but widely varying chances to maintain it.
Another thought here is this - if we'd want the saves, especially for ongoing effects, to be thrown against the caster's CHA in some way, then that means, lorewise, that the caster is actively maintaining the illusion, while also creating the next illusion, arguing a point and defending. "You are acting in bad faith." "You, other guy, i find your lack of beard distasteful (standard attack), but your buddy is still acting in bad faith, just so you know." I doesn't sit well with me. I think it makes more sense for the illusion to become a seperate effect that lingers without further support.
I am open to the suggestion of using a d12 in the same system, in that case the probabilities scale more linearly, with the worst save being an 8,33% success and the best save being a 16.66% failure on 7+. If we lower the bar to 6+, then the two numbers are reversed and we get a more success-prone system.
TL;DR - No. No summary, you should read through.
One more thing. What about non-ongoing effects? Things like existential dread can be crippling if the save is made after the effect. If we make the saves before the effect, then CHA gets very much marginalized - impotent against other high-CHA characters, and mediocre at best against others. Maybe define save times in each ability? And/or add reduction instead of cancellation for the higher level abilities?