For ten years I struggled a lot with this stuff and then Doom points from Tales of the Valiant came out. I renamed them "
dreadful blessings" in my game so I could flavor them as things like "Blessings of the Nameless King" or "Blessings of Ibraxus" or whatever.
I replace Legendary Resistance with these things and, at higher levels, I usually drop a few more onto a creature. Currently about five per big boss in high-level campaigns but I might do six at 19th level.
Dreadful Blessings are visible to the players. I use cool metal shadow of the demon lord tokens for it. That way players see them and know they're there.
A boss can expend one of these at any time to do a lot of different things, including automatically saving on saving throws but also things like ripping through force cages, penetrating resistances, getting advantage on a volley of attacks. Last night I used three at once to let the boss make three full action attacks (18 attacks from a marilith) against three targets. Once I used two so a dragon could break a stun, recharge a breath weapon, and then use it.
A big key, and the biggest complaint I've heard, is that they're arbitrary. GMs can use them for about anything. I get it, but man, at high levels, we really
need something that can work against anything because poor bosses get
hit by
everything. Just dealing with saves isn't the problem. It's huge character defenses. Skyrocketing saving throws. Huge damage spikes. World-breaking terrain manipulation. There's no single mechanic that can deal with all of that and still let a boss feel like a boss.
Dreadful Blessings seem like total cheat cards for the GM. They
are total cheat cards for the GM. But they're limited and players
love to burn them down. I regularly have players throwing save or suck abilities on bosses now just to burn down their dreadful blessings. I've surveyed about eighteen players I've used these with and pretty much to a player they're totally fine with them.
Being a good GM who's able to recognize where to use them and where not to is important. I will, for example, let a dragon be stunned until the beginning of their turn at which point they automatically break the stun. I'll let them rip open a wall of force but it takes their action to do so. It lets me tune things both so the boss isn't totally screwed but also costs them
something. It's a bit of advanced GM work to get them right, I think, but they've totally changed my game.
I'm running a currently 19th level 5e campaign with six players and three different versions of 5e at the same table and Dreadful Blessings made keeping up the challenge so much easier.
So that's been my solution.