Ask Game Masters

The problem I'm struggling with is combat that's too easy. I usually have 6 or 7 players and battles are written for 4 or 5. At some point the sheer action economy means the fights are too easy. So I'm looking for a way to make fights more dangerous without straight up killing low level adventurers (going unconscious is fine, I just generally don't kill players until mid levels) and without making combat take a long time.

So far I've tried;

Extra HP, which turned into a slap fight. Players and baddies just rolling to hit over and over. It takes longer and is boring.

Adding Enemies, this can induce a feeling of danger but as with the extra HP it takes longer because the party still has to chew through the extra HP.

Extra Damage, the most promising so far but very hard to balance. It's easy to overtune it and make a fight impossible for a party. But it doesn't affect time as much and it's been the most popular so far.

Ideas I want to try include;

Replacing a baddie with a spellcaster or adding one. This would hopefully not drag the fight out, but would require dynamic thinking, especially if extra damage is in play for the martial enemies.

Wave fights, it's easier to control for time, and I could delay a wave to give them healing time. If things are going long the extra waves just don't appear.

Control as swarms, this could probably really cut time down but if one roll misses than all attacks from the grouping miss. Which might play into the action economy problem I'm already facing.

3 round anti heroes, choose one or several enemies to stay up for 3 rounds no matter what. Record their damage and kill them off as they get hit in the fourth round if they hit zero. But allow them to remain conscious for 3 rounds. But I wouldn't be able to give updates on their health to the players. Maybe I could communicate they special status instead, like glowing eyes, or an aura of zeal.

Any other ideas are welcome!