Great Old One
And the reaction happens after that circumstance "finishes." When does "starts to cast a spell" finish? When have you finished starting?
As soon as you have started, by definition, any time after the start, is not "the start"...
As far as I'm concerned, the answer is, "When you finish the spell."
Because the start of anything only finishes when the whole thing finishes ? What's the point of calling it "the start" then ? Honestly, that argument has no ground in actual use of the word "start".
Clearly I do indeed have to put this in every single post:
I rule that a readied action cannot interrupt other actions, except for actions (such as Attack with multiple attacks) that allow the person acting to do other things in the middle; and movement, which is not an action at all.
You don't have to, since the above sentence in general is not a rules in the books, by any stretch of the imagination. Once more, the rules only do what they say they do. When something cannot be interrupted (for example, a dispel magic cannot interrupt an instantaneous spell), it is written in plain letters.
Do you have an example that involves the Ready action? Because the Ready action is the one that says the reaction takes place after the trigger finishes. If your examples are stuff like counterspell or the Xanathar's "identify a spell" reaction, they have nothing to do with this.
Do you have any example of something that cannot be interrupted ? Even the dispel magic above is borderline, the intent is not to interrupt, but it's what can be affected.
No one is disputing that reactions exist which can interrupt other actions. The question is whether Ready is one of them--and, in particular, whether the rules require Ready to be one of them.
And again, since the rules make no provision and ready actions are meant to interrupt someone's turn anyway...