Take a note of which assassin actually pulls their weight, and protect them at all costs, especially if they lack self peeling.
ETC is my main tank, and while I still engage enemy and cause disruption, I try to stay within powerslide’s distance from my healer or best assassin, so that I can always slide back to peel.
It’s funny you said that. I was thinking the other day that tank without cooldown is just a strong minion you can control.
But in all seriousness, I think even when you are out of cooldown, you can still bodyblock, whether you are denying enemy from approaching your backline, or to make their retreat more difficult.
Also, enemy doesn’t always know that you are out of cooldown. Garrosh’s throw has such a long cooldown for a basic ability, but people tend to scatter when enemy Garrosh approaches, regardless of his throw’s availablity.
Or as ETC main, I’ve had enemy blow their CC on me by simply walking into a group of them, even when my Mosh wasn’t available. Either that, or I would force them to hold onto their CC, saving it for my Mosh.
Tank is probably the only class where their mere presence can be a major threat.
I also climbed to Masters using Tanks.
First, you need to know when to go which Tank. Which can solofrontline (Johanna, ETC, Garrosh, Malganis) and which can’t, which can counter dive and which can handle poke. Which is good against what comps (Anub against Mages, Artha against melees). Which is good with your comp or not (Dibbles, Anub, Mura enables dives, Jojo and Blaze are good at defense, sitting in the front so your backline can deal dmg).
After that, you need to look for enemy mistakes and protect your allies so you are focused and not them while they can deal their dmg. Distrupt key targets and abilities. Anchor camps (you deal low dmg to camps but you can zone an invading enemy out).
But if you play Tanks, you are at the hands your allies in a way. Since you can’t waveclear well enough, can’t do camp quickly, and you’re not supposed to chase, but who knows your team will follow up on your engages or not.
Tanking imo is the hardest role. So if you’re at yourreal skill lvl, you’ll have a hard time as them in low elos.
If you’re solofrontline, don’t dive. If you’re not, that’s your Bruiser’s job.
Tanking (imo being a melee in general) is about a back and forth playstyle. You go in, interrupt some abilities, draw aggro, and you go back to your team.
No need to babysit the enemy Tank or Bruiser 0-24, tho actually, focusing the Tank is a totally good chocie to make during a skirmish fight. You all can retreat after (if you all went low on resources (hp/mana/energy)) and go back forcing a 4v5.
I suggest you watch some Tank play from high ranked Streamers on from pro league.
I’m not a Streamer, nor a pro, but here is a good Tank match I did this season:
I was Mura against a Garrosh and my main job was basically just making him throw me and jumping back.
I’d say that drafting a tank that has a lot of CC is important. Stitches, Arthas, and Tyrael are good, but they lack hard CC so with them it can be difficult to lock down an enemy vs a tank with hard CC.
I only know Stitches and Anubarak enough to take in Ranked and I can say by far that I’ve had better experiences with Anub than Stitches. I’m able to lock down heroes way better with Anub. As Stitches I feel like I’m only a meat shield that has a high health pool. I don’t know if its that I’m not playing him properly or something but the only time I can “lock down” a hero is with a good hook/gorge combo. With Anub I can lock down a hero every 10 seconds or so.
Secondly, I just want to say that your mindset has to change from being an Assassin/Bruiser to being a protector. Your job isn’t to kill, its to allow your team to kill. You should not dive into the enemy team leaving your team unprotected, even if that means you’re just walking around waiting in front of your team. Playing defensive will make the enemy team second guess their decision to dive in and when they do, punish them.
Also out of everyone to keep alive, keep your healer alive. You and your healer are the most important on the battlefield. Without you two, your DPS will have a hard time getting kills or staying alive. You protect your healer and in return your healer will keep you and your DPS alive.
The latter is always more important than the former, no matter what people say.
As a tank, it’s critical that you use your health and your CC to protect FIRST. If people need to be chased, only do so if you can still ensure your team remains safe in the meantime.
If you want to dive the enemy team with a large healthbar, play bruiser. Tanking is a tiring experience but unfortunately an important one since most healers and DPS have heavily limited peel.
Yeah stitches is the worst tank
party up with ppl you dont have to save constantly 
i mean if they are constantly overextended no amount of good tanking is going to help you.
…but this is why i dont want to tank or heal anymore. i am pretty good at both i just get really annoyed by the ppl. so i only assassin as much as i can.