All of the Demon Lords pretty much already are viable for a group of level 20 characters.
Very little is needed to be modified with them in order to prevent them being cheesed.
That may be true if you're not allowing magic items. I'm allowing magic items. I've found Demon Lord damage output and hit points to be relatively low. I'll be boosting hit points. Adding Damage Resistance against magical weapons to make them stand out from regular demons. And adding knockback resistance to cantrips after the fun discussion on
eldtrich blast knocking enemies back up to 40 feet. I can't be having high level warlocks knocking demon lords back up to 40 feet using an unlimited use cantrip while the archer and ranged attackers pepper them with powerful damaging attacks turning a demon lord fight into a joke of an encounter. Just as I modified the
wand of viscid globs not to work on legendary creatures because restraining a dragon or demon lord for an hour while the party whacks on it is an incredibly stupid and anticlimactic battle. Mearls wrote it off as not a problem. I'm wondering how much of a problem he'll think it is if he gets play reports back from
Out of the Abyss that it was a cool adventure until parties started using the
wand of viscid globs to restrain demon lords and do massive damage from range ending them with far more ease than I hope was intended. I've cleaned up some of that stuff to ensure it doesn't end up being exploited by my min-max players.
The one thing I've always liked about playing with min-maxers is as a DM you learn how to counter some of the most devious tactics the game offers. You learn how to build monsters to last even against tactics that usually trivialize encounters. I've had to make a lot of adjustments to deal with such play over the years when one of my players comes up with some crazy combination that renders an encounter trivial and tries rinse-repeat mode to "break the game."
The demon lords may be ok against a lvl 20 party without magic weapons or only a few magic weapons or a party that isn't particularly adept at tactical play. But DMs definitely need to pay attention to the
wand of viscid globs. One hit of that wand and you have a restrained demon lord for 1 hour. The restrained condition is rather harsh. The only demon lord immune to the restrained condition is Juiblex.
If you have a warlock in the group, I'd watch out for the Repelling Blast reverse kiting tactic. Spread a party of heavy ranged attackers out so the demon lord can't focus on one. Have the warlock ready an action as soon as the demon lord closes range, hit him
eldritch blast knocking him back 40 feet to ensure he can't melee attack. Each time the ranged attacker gets his turn he falls back 30 or more feet to ensure that the demon lord has to continually close the distance using 20 to 30 feet of movement, then knock it back 40 feet so that it can't move to get all of its attacks or melee legendary actions to keep the damage numbers in your favor. It's a very effective tactic if you can set it up.