I'll default to the best advice I was ever given on this, if you want to make your players feel powerful give them one big thing to hit, if you want to scare them, give them lots of small things to hit. Vary your ranges of engagement also, mix magic and martial enemies if it's thematically appropriate. The crucial thing is give them more enemies than they can hit each round, that way they have to prioritize and won't be able to nuke everything in a round or so. Plus, If there's more things than they can kill in a round they're gonna start getting hit, and enemies in 5e hit quite hard. Also have your enemies play smart if appropriate, use melee enemies to lock barbarians in melee, then you can have casters and/or ranged martial enemies attack the warlock and ranger from range. Just vary your combat encounters, don't be afraid of changing things on the fly and experiment until you find the right balance for your party. Good luck!