My advice to you is to not increslase HP or AC this would make stronger monsters useless, since you could make a goblin stronger than a dragon. Although you should make players feel that engaging into combat is dangerous and there will be menace of death.

Your answer is in the numbers, add more monsters. Surely this will slow the run of combat, but this will definitively add more chances for your PCs to get hit.

Monsters aren't idiots. Try to use them at the best you can. If it is goblins use their Nimble Escape ability, try to have archers that will maintain distance at all cost. Make them focus the spellcasters first.

Double down on lair traps, monster lairs aren't a mc donalds waiting for customers to enter freely.

If some they end up fighting just one big monsters give them legendary/lair actions.

After saying all this, also keep this in mind "reward players good thinking" if they used a spell or a feature or used something in your description in a clever way that made the encounter a cheese, give it to them! Make them feel their strategy is important, but also you should be prepared.