HAPPY EASTER!
This is a bit confusing. The chart at the top of your post shows that the length of Eastertide is from “Easter Sunday - Vespers of Trinity Sunday”. However, most sources that I have read (including one or two that you link to later in your post) state that Eastertide runs for a length of 50 days, from Easter Sunday to Pentecost Sunday. The chart at the top of your post seems to have added an extra week to Eastertide that most sources do not.
So which is it?? I have to replace The Angelus with the Regina Coeli during Eastertide, according to my New Saint Joseph Baltimore Catechism. Hence, I need to know if I should keep doing the Regina Coeli until Trinity Sunday, or if I should switch back to The Angelus after Pentecost Sunday - a week before that.
It can all be very confusing at times!!