You have two jobs as a tank: control space, and manage the party's HP reserves. to do these things you need to be hard to hurt and harder to ignore. Low AC to temp enemies to attack you can contribute to this, but high hp isn't enough to offset that. For that strategy to work out you'll also need ways to reduce the incoming damage, like damage reduction (barbarian) or self-replenishing temp hp (fiendlock) or the like. Without something like that, easy to hit with lots of hp is just a drain on the healer's spell slots, which unlike in an mmo are limited per day.
But yeah, you do need something extra beyond being hard to bring down to make you difficult to ignore in order to be a tank and not a turtle. Lots of options out there, whether through spells, feats, or class features.
My favorite tank in 5e is conquest paladin, and their main thing is to inflict frighten on one or more foes with Wrathful Smite, Conquering Presence, or Fear, then move to bring enemies that fail their saves within range of Aura of Conquest. Such enemies now have disadvantage in attacks & checks, speed zero, and take damage every turn. Melee enemies can't move to target your allies at all, Ranged attackers can do so, but suffer disadvantage, and are generally desperate to deal with you first regardless.
You won't be able to lock down every enemy this way, but even just 1 to 3 will often be half or more of an encounter's fighting strength, breaking up the overall encounter into chunks that are easy for the rest of the party to chew on one at a time.
And if the entire enemy group is immune to frighten or passes their saves, the conqueror can still fall back on the standard paladin tanking kit - Bless (a major buff that enemies are often eager to bring down), Compelled Duel, powerful opportunity attacks (enhanced by Divine Smite, Sentinel Feat, or maybe Warcaster/Booming Blade if you multiclassed warlock or sorcerer), or just high smite damage targeted at the enemy leader. Your ability to play secondary healer outside of combat also helps mitigate hp reserves.
My second favorite 5e tank is Battlemaster with sentinel, polearm master, & tunnel fighter. Other fighter subclasses also work but note that cavalier specifically suffers some bad antisynergy here due to reach issues & bonus action economy. The combo also works for paladins in theory, but the dual feat combo is very taxing given the heavy pressure on paladin ability scores & asi's due to Madness & lacking the fighters bonus ASIs. Vumans can make it work, but it's still taxing.
The combo requires UA, & extremely old & deprecated UA content at that, so most DMs won't (& really shouldn't) allow it, but if yours does then unlimited, movement-cancelling opportunity attacks against enemies crossing the perimeter of a 25' diameter control zone in either direction is pretty amazing.