How to introduce this to a person who is willing but not enthusiastic, without losing them in the first month.
The failure mode is always the same. Somebody who loves this hands somebody who does not a list of thirty-five films, and the list itself is the thing that ends it.
Do not start with a list
Start with one film that works on its own and stop. Iron Man is the obvious one and the best one for this: it is a self-contained story about an arms manufacturer, it has no homework attached, and it ends properly.
If they want another, you have a viewer. If they do not, they have watched a good film and nothing has been wasted, which is a much better outcome than abandoning a project.
Let the second one be the payoff
The Avengers is the film that explains what the whole idea was for, and it works with almost nothing before it. Going straight from the first to the fourth is a real shortcut, and it is more persuasive than watching the two in between.
Nothing between them is required to follow it. The one-evening path on this site is exactly these two.
Then let them choose
Once somebody is in, hand them the timeline rather than an order. Let them pick the character they liked. The catalogue is built so that following one person through it is a legitimate way to watch, and the watch paths on this site are several versions of that.
What to avoid
Do not explain the phases. Do not start with story order. Do not put a series first. And do not tell them the surprise in the film you are about to watch because you are excited about the one after it, which is the most common way this goes wrong and the hardest to resist.