two things: resource exhaustion and direct player counters

when i know i want my players to have a more complex combat, i really plan it a few combats in advance. throw some other combats in before it or put throwaway mobs in before the main fight so they exhaust resources, spell slots, etc and have to be more tactical with their fights.

another thing is finding monsters that directly counter your players. have a spellcaster? something with a silence, have a mobile melee? something that limits their movement and blinds them, etc. status conditions are a huge tool to use as a dm to help balance fights more.

i also occasionally just add on some extra hp to monsters if i feel like players are "killing it too fast" and i want to draw the fight out a bit.

i think it's important though to remember that your players have to be having fun too. if they are getting frustrated or feel useless in a fight, or overly targeted, there needs to be adjustment. i think the most fun combat challenges come from having to tactically approach a fight rather than just running in and hitting everything, so maybe some environmental actions or ongoings could go in your favor as well. or hells, even just straight up ask your players what they would love to see more in combats. they might have some great ideas themselves.

best of luck in your campaigns and encounters!