The risk in 1 & 2 is that monsters become a slog and characters are no longer heroes. If you need to hit a monster 30 times before it dies (because half misses due to AC, and you need a lot of hits due to HP), it's just slow and boring.

Lair actions and distractions/barricades to get to BBEG are where its at. There's a trap in the way. New support keeps popping in until something is destroyed. Something changes drastically halfway through the fight.

1, 2, and 3 are easy options, but lazy AF and deeply unsatisfying in most cases.

4 and 5 is really where it's at, but it's quite a bit more work. 4e's early maths was really bad, with enemies becoming enormous damage sponges, though they were otherwise probably better than 5e. Late 4e improved its monster design somewhat. Pathfinder 2e is great, so borrowing monsters from that could be good. Or Matt Colville's books for 5e.

I'm a fan of the extra HP.

Extra AC and damage can be a slippery slope tho.

Look at older DnD editions and see if the monster or any similar monsters have extra abilities you can add

This. The Tarrasque had so many anti-cheese abilities in older editions. Now its best anti-cheese is running away, and even then there's ways to keep up.