You're absolutely right with everything above, but the points you bring up actually prove my point that tank items are inherently inefficient.
So like you mentioned buying 1k bonus HP might only be a 33% increase in effective health, but you have to get your hands on some raw HP early because it's the only thing that works agains both AD & AP damage, and you've got AP champs going for pen boots + Morello early on. So until you get some big MR items fully built you need that HP. Additionally you've got fighters taking Conqueror for the true damage, so literally HP is the only thing that matters in that scenario. But then you like have to build Armor & MR too cuz you've got so many items and damage sources doing %hp now days.
So whenever someone procs Conqueror all your resistances are literally meaningless. Then when you look at armor items specfically, they are pretty bad against anything besides the ADC. Deadman's Plate is not in a good place, Firecape damage is negligible after like level 10 so it's effectively just helping you with your push a little bit. Three of the best items for armor stacking are Randuins, FH, and Thornmail, but guess what? Their attack speed passives don't stack, so you have to make your choices and gimp yourself in some fashion.
Every other role in the game becomes more effective as you complete items. Allowing them to synergize well together. For example a mage will likely go Ludens or Archangel first, then they immediately have a bunch of options. If they are doing ok, they could go Morello for the early game power spike, but if there are tanks already stacking MR it might make more sense to go straight Void, but if there are juggernauts on the enemy team who usually stack hp then Liandries might be the buy. Then with your 3rd item you can take like Zhonyas or Banshees for a defensive item that still gives you damage. That will likely get you to 40% cider if you aren't already, but even if you are it converts to AP with proper runes. Same deal with ADC. You Open with a damage + crit or on hit item, then generally go into a zeal item. You have several choices depending on the situation, but no matter the choice the attack speed + crit will scale well with your first item. Tanks right now are super gimped because their itemization is just inefficient. They have super limited options. If I build Fh as my second item because lets say the enemy team has a Irelia and a Xayah, then I want Randuins too for the crit negation and the HP for Conqueror. But guess what? The attack speed passive is wasted. A waste of gold. And then in the early game Aftershock and Grasp are actually pretty important for survivability, but they have CD's so... And if I'm not playing a tank with self-healing, then Adaptive might be my MR buy that game if I'm playing against Cass, but if no one else on the team is a spam caster or on hit damage then it's again, completely wasted passive in any fight where Cass is not present.
TLDR: Riot needs to take a very close look at tank items and think about the situations where people would want them, the order they typically buy them in, and what the most desired combos are. And find ways to optimize those combos and open up options as widely as other roles have.