I'm not going to be able to give well thought-out crunchy observations and recommendations that many others in this thread have, but I've learned some things over the past two years of being a DM for players with FAR more experience in gaming than I, and comparing that to Adventure League games.
1. The game is designed for 4-6 players of casual players. It seems to go out of its way to not make things too deadly.
2. Experienced players will not be particularly challenged if you go by the DMG CR system, unless the dice are against them. Most AL games I've gone through would not be a challenge to my players. The only exception was the Epic game I played recently, which was brutal at tier one...but you are supposed to go begging higher tier tables for help in those games, so its a completely different experience.
3. I don't like to just give more hit points or more lair actions, etc., unless I'm building a new monster or there is a really good story reason for doing so. I like to play the game RAW. Instead, I've learned to try to put myself in the big bad's shoes. It is silly that so many powerful, sentient creatures, with strong wills to survive (especially those that are supposedly hundreds or thousands of years old), are so dumb in terms of preping their defenses.
4. With my players, I find that I still underestimate them. I will prep for a battle for a long time, worried that it is going to be a TPK and it rarely turns out that way. I keep the kid gloves off now. I have *finally* had some encounters where players have run away from, which is nice. The party shouldn't win every battle.
5. Sometimes, it is fun -- if you have the right players -- to build an opponent that so over powers the party that their goal isn't to defeat it, but instead to make a quick raid, or stealth in, grab a MacGuff'n and get out alive.
6. Always have some things in your random encounters that are undefeatable and require avoidance/escape. One, isn't that more realistic? The world doesn't level up with you like a video game. Also, if the party is used the occasionally having their buts kicked or having to say "oh HELL no! RUN AWAY!", even seemingly simply battles will be taken a little more seriously. If they know that you are not tailoring encounters to them using CR, they will learn to engage with care, preferably after some observation and analysis.
7. When you DO have a big bad that goes down "too easy", well maybe there is more to it. Maybe that demon they summoned was so easy to summon and trick because another demon was "helping" the party as part of some infernal power struggle. Not only do they now have an unknown infernal benefactor, perhaps the summoned and destroyed demon's superior is now pissed off at the party. I mean, come ON, you don't dabble in demonology and expect that you get away with attacking an officer without others in infernal hierarchy taking notice. The party is now either a threat to be crushed or a pawn to be used. Devils are are not scarry because each individual can do X damage per round, but because there is an ENTIRE HELL of these immortal mafia-nazis you might now have to worry about. Demons are scarry because of the hordes of power hungry eternal-psychopaths that want to see the world burn and your killing one of them may be part of that. A group of 5th level characters simply DO NOT KNOW WHAT THEY ARE GETTING THEMSELVES INTO when they start messing with demons and devils.
My groups has spent several months sneakingn around and avoiding an entire kingdom because they betrayed a Rakshasa. Sure, if they cornered him in a room, they would take him out without much sweat. But this is someone that has minions and spy networks and centuries of experience building and protecting his power, who commands armies, and plays neighboring kingdoms against each out. If they ever actually go after him and actually reach him, it would be anticlimatic. The challenge of killing Hitler is not the fight he would put up, its getting to him in the first place. Stop trying to make every big bad into a tarasque. The tarasque is one of the most borring monsters in D&D. You plink away at it until it dies.
The only way I've found making a bucket of hitpoints interesting is to have something at stake that make the party need to destroy it in a certain amount of time (before it reaches the capital city).
Sorry, I got long winded. In short, I think it is time another demon/devil approaches your party with a "deal they can't refuse."