Best way to scale difficulty?

I am running Stonehell pretty much as is but in Shadowdark with my 6 players. It seems to work perfectly with no conversion, but the encounters just don’t seem difficult enough. I suspect it’s not the module but rather the number of players.

However, if I were to simply multiply the number of enemies by 1.5, I think it would just get “swingier” and not more difficult. On the monster turn, they would easily swarm and probably kill one player, then there would be 5-6 turns of the monsters getting pounded, back to maybe one more insta-kill on a player.

Upping the HP of monsters also seems like it would just drag out combat further... However would you handle this situation?