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Or you could just roll initiative as normal and stop making it complicated.
I can easily imagine circumstances where initiative would be rolled before the door was opened, and circumstances where it wasn't, but it's never complicated. The inclusion of reactions in the action economy keeps it simple.
If you're really just hell bent on putting any readied action into iron-clad initiative context, then when the party opens the door and you roll initiative, you begin on the turn of whoever opened the door. Anyone with higher initiative can have an attack held and make it after the triggering action, anyone lower can't. That seems more complicated to me, and unnecessarily limits the people with the highest initiative, but I suppose it's workable.
What's not workable is a scenario where Mr. Armor Pants tromps down the hallway making a huge racket, giving the clever goblins plenty of time to take cover and knock arrows. When Armor Pants kicks in the door to the clever goblins' room, you roll initiative. Twinkle Toes wins, and she prances into the room, detonates some potent AoE on the poor goblins, and prances back out into the hall. By the time you get to the goblins' turn, they're all dead, having never taken a shot, because despite being aware that the party was coming, hearing them outside the door, and having arrows knocked, you felt like they couldn't ready an action until after that door got kicked in and initiative was rolled.
Sure, you can handle that by rolling initiative before every door is opened, but that's a lot of unnecessary initiative. If the party knows the goblins are there, you can roll initiative before the party is anywhere near the door. If the party doesn't know about the goblins, though, letting the goblins use their reactions when the door is kicked in or to be holding an attack waiting for a target to present herself is by far the most elegant way to handle the situation. It is, in fact, what reactions were designed for. It's simple, not complicated.