How can a support be an effective tank with 2k hp??

Hi, i think there is a serious issue with pure tank supports like Alistar, Braum and Tham kench.You know the ones that only build defense and not a single point of damage. The core build for those champs seems to be zeke's convergence, knight vow, iron solari and sometimes righteous glory or gargoyle stoneplate wich give you a very unimpressive 2k-2.5k and maybe a 3k-3.5 k hp if you reach full build while some champs like zac,ornn or sejuani can easily reach the 5k hp ...

When i play a tank bot i feel super squishy for most of the game unless i am super feed or when im build done... As braum if you facetank a not even fed jhin ult with your shield to protect your adc you are still gonna take an hell of a beating ( like 75 % of your hp ) ... alistar is gonna go in and die in like 2 second cuz he is gonna get destroyed by a darius ult, garen ult, vayne silver bolt, ahri Q, velkoz ult pyke ult master yi E ...( PLZ RIOT MAKE ALISTAR TAKE REDUCED TRUE DMG DURING HIS ULT !!! )

I'm convinced we need better tank items for support but those items should be accessible only if you have a ward item purchased to make sure some champs like darius or xin zaho can't buy them because i remember a time when champs like xin or nocturne were all buying locket of iron solari because it was super cheap and because it was giving like 500 hp, and armor and mr aura for the whole team and the shield active wich made them super tanky while dealing absurd dmg ...

There is actually 4 or 5 tank support that are actually  good and would be broken if we go on with those changes : Leona, Nautilus, Blitzcrank, Thresh and Taric, The reason why Nautilus, Leona, blitz are thresh are actually good is because they almost have enough damage to almost kill an ennemy adc or mage without any help when they go all in with their engages so i think a damage nerf should balance them. For taric I don't know... his ult and his insane self healing make him very hard to kill so everything i said in this post doesn't really apply to him ...

The other option i see to fix the tanky support issue would be to give the weaker support some major tankiness buff

-Alistar could get like 1 or 2 extra maximum health everytime he proc his triumphant roar and his ult should REAllY aplly to true damage. Taking a 1.2k true dmg dunk from darius when you have 2k max hp and your ult on make me wanna throw my pc through my window ...

-Braum wall could work like yasuo and block everything for like 2.5-3 second ...

-Tahm kench Q could be able to go through minions and hit multiple champions ...