Enemy Balancing Help Needed!

I've been DMing a decent while now and I'm having a blast but one thing I'm consistently really struggling with is balancing my enemies.

I currently have a party of seven level 5 players - Monk, Warlock, Paladin, Druid, Barbarian, Wizard and Sorcerer. So naturally they stomp every encounter. Obviously I've bitten more off than I can chew but I'm not willing to give up yet.

They're not min-maxing players but the sheer number of them, plus their combination of ranged and close ranges has made them quite tough to scare.

How do I go about giving them a real challenge? I don't want to throw a dragon or something at them every session, I want to be able to have even grunts and thieves to give them a bit of pause. My question is how's best to do this? Any custom tweaks that work for people? It seems no matter how much health I give the enemy, they shred them.

Should I buff the AC to numbers like 18+? Should I give them 2-3+ attacks? Do I buff the damage? And if so, to what? Should it just be a case of tons of enemies? I'm struggling with what to do here, if anyone has any tips, I'd be greatly appreciative.

Thanks!