Fun factoid. Before World War 1 passports, as we know them, did not exist through nearly all of the world. They were a 'temporary security measure' like so many things... It wasn't until 1978 in the USA that entering or leaving the nation requiring a passport was legally encoded (though we'd been using them since the early 19th century). That's why reading of e.g. scientists from before the early 20th century you'll regularly see them being born in one country, educated in another, opening a business in another, and so on.

The reason I'd agree with your timeline is one thing you did not mention -- wealth explosion. Purely recreational international travel is and always has been relatively expensive.