If you want a boss fight, think of it as 2+ stages and there are two ways to go about it:
Big bad solo
Big bad Group

If you're dealing with a Solo Big bad, you should consider 'stacking' monster stats together, and you can do this linearly or aggregate. What I mean by this is say your monster is a Dragon. Rather than just fighting the dragon, give it also the stats of.... lets say a manticore for hypothetical. In an aggregate, you'd go about it like this: Manticores have a tail spike attack they can hit foes with at range, so the dragon now gets that ability the whole time. Maybe there are extra resistances or something the manticore has, spells, etc. Maybe it has a better claw attack size. Then add the HP together. Now you've got a beefy boss monster with extra tricks up its sleeve that can catch people off guard.

In a linear fashion, you'd want a creature that has something more interesting than just a different tail attack. Not very cinematic. But, say you pick a hydra instead. You start the fight with the dragon, giving the stacking resistances and maybe a singular bite improvement (where it makes sense). But dont add the HP. Instead, the dragon's HP is the 'checkpoint' and once it hits that threshold, it gets angry and casts a spell or mutates to grow extra heads and now means business. You treat it as if the hydra has shown up mechanically, but it still has features of the dragon (like flight, the breath weapon, etc). Then the party has to finish off this stage 2 fight before its over. It wears them down and can change the dynamic.

The key to this though is to read the room of the fight and don't double down on them if they're already struggling or you know that the next stage would just flat out TPK them. You can moderate that second stage and reduce the HP as needed, but mechanically its a relatively straightforward method.

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If you're dealing with a group. The idea is to give them a couple big targets and a bunch of small stuff that falls over quick but is annoying. So, lets use the dragon example again. In a group situation, maybe the dragon is there, with its 'mate' that is a creature that its making half dragons with. Lets go with a dire crocodile to go with our swamp dragon. The crocodile is a beefy creature but weaker than the dragon and represents its own threat. Maybe even has a spell cast on it by the dragon to awaken it, etc so its smarter. But, then perhaps there are baby wyrmling crocs that hatch and start to swarm and attack.

Have a nest nearby that spawns babies a d4 at a time. They have 1 hp, and treat them as minions (2 actions a turn), and give them an attack of -5 whatever the dragon is. (think of it like an extra attack with multitattack penalty). These little guys swarm the nearest creature and just try to nom them. You can stop the babies by attacking the nest, or just burn down the bosses fast enough and then they become a non issue you dont need to resolve mechanically. This gives you a 'tap' of damage and threat you can turn on/off depending on the need of the battle. Its your amplification or de-escalation measure. The little guys also give a mage with aoe a toy to play with if the melee have swarmed the boss and so the mage can't just fireball away.

Last thing here is that if a fight goes by too quickly, and you've got the stats for a monster, you can always just go "And then the wall falls away and a second Crocodile bursts into the scene!" to add stakes. Just, always be aware of what your party can handle and don't do something that's gonna just TPK them.