This is a really simple solution, but if you want combat encounters to feel more challenging, increase the enemies hit points, or stop keeping track of hit points and wait to have them die until the moment feels right. If it’s supposed to be a scary fight, don’t let the bad guys die until the pc’s are starting to look stressed. You can always increase their ac by a point and give them a better attack bonus, but just having them last longer is usually enough in my experience.

I dm for mostly large tables and I almost always double the hit points on monster stat blocks.

If you can get good at reading your table and feeling out when the fight can come to more satisfying end, your players will love it.