Good to hear, Dave.
I haven't yet decided if I need or want to make an adjustment, but there is something in my blood that wants a CR 25 demon lord to be a serious threat (like 75%+ chance of TPK) to a lvl 20 party of 4 of optimized characters (without epic boons). I guess I would like to see what an optimized lvl 20 party of 4 could do and go from there.
I honestly think the best way of thinking about it is the coldly analytical sober way:
Q. What is a CR 25 creature to a Level 20 party?
A. A creature five steps above their level. Nothing more, nothing less.
Meaning that I'm thinking you want different... layers, tiers or what have you.
A CR 25 Demon Lord is supposed to be more "epic" than a regular vanilla CR 25 monster.
But I don't think that's the best way to look at it.
All CR 25 critters are epic. That's why their CR is 21+.
Whenever you want strong lieutenants for your epic Lords of Doom, use CR 11-20 critters.
But resist the temptation to add in rank-and-file CR 21+ monsters. The game basically ends at or near level 20, and so anything above that CR should be unique or nearly unique.
That means accepting that a CR 25 demon lord will be somewhat of a threat to a level 20 party. Not more, not less.
How big a threat? That depends on the party optimization. If you ask my party or perhaps Celtavians, they will steamroll a single monster of APL+5 challenge rating. But I'm sure you can easily imagine another party that would find a monster at CR = APL to be exactly what the DMG assumes, that is an "average" encounter.
Rather than messing with the CR system, I think it's useful to stick with the same compatible "CR unit" used by the Monster Manual and the adventure supplements.
And instead accept that for a Demon Lord to be a serious threat to a minmaxed party of level 20 characters all by itself, it's CR should probably be 35 (using WotC's CR units).
You should probably keep the good work with your extended monster statistics table, where you roll back the way WotC made AC and other stats slow down as you approach CR 20.
But I honestly think you and us, your readers, would get the most usefulness out of your monsters if they stick to the only-lightly-tweaked WotC challenge ratings.
You could still keep ideas like the epic bonus. It's fair to see new curves appear at the start of the epic 21+ tier, just like PCs get revolutionary capstone abilities at level 20 (at least some classes do

).
If you extrapolate the WotC CR progression up to CR 40, where would your Juiblex DPR fall on that scale. Let's guesstimate that kind of damage would mean CR 32.
So be it then, let's give him CR 32.
That number expresses, to me, that a level 20 party could take him, but only if they play at the top of their game.
Which probably is exactly what you want.
To the "average" WotC-assumed Dungeon Master, CR 32 says "instant death trap far far more dangerous than the CR 20 monsters we can handle", and that would be fine.
Everybody would keep speaking the same language.
Your CR 23 critters wouldn't have to be "strangely" much deadlier than the ones WotC publish, and the ones published on DMs Guild.
Because they wouldn't be CR 23. They would be CR 32.
