[RPG] When can you counterspell a spell with long casting time

The spell counterspell in D&D 5e has a casting time of 1 reaction, "which you take when you see a creature within 60 feet of you casting a spell". It has the basic effect of:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

Most spells have a casting time of 1 action or less, in which case the timing your counterspell shouldn't affect anything outside of weird corner cases involving multiple reactions on the same spell. I'm not asking about those corner cases. However, some spells have a casting time longer than a single action, such as alarm with a casting time of 1 minute (10 rounds).

Alarm is a 1st level spell so should be countered by counterspell without any need for a check. However, I'm not certain what time within the 1 minute window you can actually do so. Specifically, all other reactions I can find specify a "trigger" that is valid only for an instant as far as D&D is concerned; there's no noteworthy duration on "hit by an attack" for example. The general description of a reaction in the basic rules state

A reaction is an instant response to a trigger of some kind

While the more specific rules under spellcasting say

cast in response to some event.

And the rules for readied actions, although not applicable here, give a "use it or lose it" precedent

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger.

All emphasis mine.

As long as the triggering events are "instant" there isn't much room for ambiguity, but the "trigger" specified for counterspell can be interpreted more as a "condition" or "situation" because "seeing", "casting" and "being within 60 ft" are ongoing rather than instantaneous. I can think of a few ways to reconcile this:

• You can only counterspell when the casting starts, which is the most similar to how counterspell interacts with readied spells.
• You can counterspell at any point during any character's turn as long as all the conditions are met, which is the natural reading but (in my opinion) doesn't fit with the concept of reactions.
• You can counterspell only at the end of the casting. This is closest to the "readied action" rules of "after the trigger", and it also causes a fairly natural distinction between getting countered, which consumes the spell slot, and getting their concentration broken which does not.
• You can cast counterspell at the moment you see the caster. Interpreting "when you see" as an instantaneous event happening right when the caster comes into vision removes the ambiguity completely, and seems (to me) to at least be a valid English reading of the phrase if not the most natural one. It has a lot of weird conceptual consequences though because the spell is counterable through its entire casting, but for any individual it only happens once.
• You have the option to cast counterspell each time the caster takes the "cast a spell" action, but not arbitrarily at other times in the round. This resolves the weirdness of "actively" using a reaction at arbitrary times, while still allowing the entire casting time to be available for countering. I like this interpretation the most, but I also think it has the least textual support.

Which of these is correct? Are there any other reactions where their "trigger" can be valid for more than an instant, which might be used as precedent? When am I able to counterspell a spell that takes more than 1 turn to cast?