Tanks need to be tankier and deal less damage

Differs from champ to champ.

There's some tanks who dealing moderate-high levels of damage is acceptable for and some who generally shouldn't deal damage at all.

Most jungle tanks fall into the first category, if you take away their damage you render them unable to jungle properly and therefore pretty worthless. Toplane tanks generally would sit here too since they generally need their damage to not get crushed brutally out of lane.

There are some tanks who feel like they deal more damage than they should however. To me the best example is Alistar. His bases are surprisingly high for a champ with other strengths as absurd as his and have been a larger part of the reason why he's been a competitive staple than most people realise. Combined with the out of place AD on his ult he becomes a surprisingly large damage threat for a tank support.

There's also champs who's methods of damage can feel wrong. Malphite is a good example of this IMO. He's already a high engage champ and him having almost all of his damage frontloaded as he does currently makes his engage overwhelming but leaves him no real fallback after. It also makes his lane odd, to say the very least. If Malphite had some of his damage backloaded rather than 90% of it being burst you could probably justify giving him tools to do something other than just hard engage, but as it stands you really can't due to the shear power of his engage.

The basic answer to your question is no, you really can't. You can't just look at a group of champions as diverse as tanks and expect them all to have problems that are solvable with a single answer. This school of thought may be a legitimate one when applied to someone like Tahm Kench but if you try to apply it to Sejuani in the same way it would destroy her as a champ.

Depends on your definition of tank. "any champion with reliable soft CC who builds HP" or "a champion with high amounts of utility who builds resistances".

Only one of them is an actual tank.

Maybe Grasp of the undying should be inverted to 1.5% damage and 3% heal (currently it's vice versa). I feel like this mastery is the source of the tank dps. Also fervor.

As soon as you give them ACTUAL tank stats. they just buy bruiser items and face roll.

1. Tanks need some base level of Damage, either to lane. or for camp clear. A Tank in lane most likely won't have the needed damage to duel tanky fighters, but Tanks like Sion and Maokai, offer enough AoE damage, that they can at least survive in lane. And for Jungle tanks, they need clear speed to match their opponents in gold and XP. They only tank who wouldn't care about damage is the support.

2. Juggernauts are probably going to share the upper tier of defenses with True tanks. And Riot Probably won't push that upper bound much higher. And even if they did, there's likely an Upper limit to how much that extra defenses actually helps.

3. Unlike Juggernauts, True tanks should generally trade that DPS, in favor of Engage, CC, and lock down. And that should be the Tanks key feature. And if Tanks have no place, against fighters, even when they have similar defenses. Then it's probably the utility that needs to be double downed on. (Like say, lower cooldowns on abilities.)

You mean bruisers?