Even with playing just the monsters in the book, high-level encounters in D&D will be incredibly swingy because there are a lot of abilities for both PCs and monsters where one character rolling well or poorly can completely change whether the fight is a near-TPK or a cakewalk. The party succeeded on the save against the cool boss ability, well they're probably gonna be fine. Or they all failed, well now they're fucked. The boss failed the save and now it's paralyzed, guess you've pretty much won. Or it succeeded, now you've wasted your turn. That kinda thing.

That's why making encounters with a bunch of swingy abilities can actually tip things back into being controllable. If your boss is getting whomped harder than expected, he gets desperate and breaks out the super-kill abilities. Or if the party is the ones getting clobbered, maybe the boss gets overcompetent and doesn't use their super-kill ability until it's too late.

I've found that at the end of the day, the PCs are generally expected to win, and they have a major advantage: when they hit zero, they get to roll death saves and can be healed, whereas the enemies usually just die. This is actually a huge factor in their favor, and explains partially why PCs can beat enemies that might seem way above their level. So honestly I don't worry too much about making strong enemies, my parties can usually handle them. And if they handle it too easily, it's not the end of the world.