they already have the normal door limitation of not being usable while in combat, so i think if you have already been out of combat, and hit the button to open the door, you should probably get the immunity while the animation is processing because its like a 2-3 second window before your character fully disappears due to the load screen (your shows the open door and cuts to black and theres still about 1 second after that when you can still be attacked)

when you get to the other side, you already have the immunity while you are loading back in

As garion said, there's ways to prevent being ganked at a door. Make sure your buffs and hots are up before activating the door, and make sure you're on your bar with your burst heal. If you can get ganked consistently from 100-0, it means your build is too squishy. Change your CP around, slot more health, or put on some reinforced pieces.

Different armor pieces give different amounts - prioritize reinforced on the chest first, then the head, legs, shoulders, and feet next. The gloves and belt are the least amount of armor, so if you need to have 2 pieces of light armor for whatever build you're running, these should be the first pieces you choose to be light.

They may be expensive, but the easiest way to boost your health is by wearing Prismatic Defense Glyphs on ALL your armor. Why not straight health glyphs? Besides the fact that Hakeijos are simply the most stat dense enchants, you want to have enough stamina to break free and dodge roll/block as well as enough mag to cast burst heals whenever you need them. Prioritize Head, Chest, and Legs here as they give the most enchantment value. If you can spare your jewelry traits, running Triune on all your jewelry is another good way to boost your health. Have a shield ability? Cast that along with your armor buff and hots before activating the door. If you're a Warden, be sure to have your toughness buff up.

Major Evasion can also help a lot. It won't help with every gank, but if the nightblade uses any aoe damage in their combo it'll greatly reduce your incoming damage. Everyone has access to major evasion nowadays, whether through running dual wield, 5 pieces of medium armor, or getting it through a gear set.

Will you get ganked? Sure, but if you have ~35k (or more) health, and ~25k (or more) armor, you'll be hard pressed to find nightblades that can 100-0 you in a single combo. Not saying they don't exist (they certainly do), but the vast majority of nightblades won't be able to pull off that much damage before you're able to break free and pop a burst heal.

Your suggestion won't solve your problem.

Vampire invisibility is also a thing that exists for all classes to utilize if there're willing to suffer the drawbacks of being a stage 4 vamp. Some pros is becoming tankier as your health goes down and being able to move at normal speed while crouched.

You shouldnt loose controll over your character while beeing attackable. Most players dont like the tank meta, 35k hp is very tanky and shouldnt be a requirement to not get killed while using a door and not having controll over your character. Getting killed without counterplay is stupid and should happen as rarely as possible. I also dont like that the doors dont work for 30 seconds after you got damage. If you get attacked you can go nowhere to escape. Most fights in IC are ganks, usually Xv1 ganks, so the victim has no chance to win the fight and I dont think the gankers deserve the kill. Some of the spawn points are also near doors, so spawn snipers can prevent you from using doors without beeing attackable themselve.